2021 Live Review 6 | AP Physics 2 | Geometric and Physical Optics

Summary

In this AP Physics 2 review, we delve into the intriguing world of geometric and physical optics, covering essential concepts like the electromagnetic spectrum, polarization, and the wave-particle duality of light. The session explores how light waves interact with various media, emphasizing polarization, refraction, and diffraction. Through simulations and engaging examples, students grasp how light behaves differently when it encounters slits and thin films, offering insights into its wave nature. This session also touches on the practical aspects of lenses and mirrors, explaining how they create images through refraction and reflection.

Highlights

• The electromagnetic spectrum is like a grand rainbow, ranging from radio waves to gamma rays! 🌈
• Polarization proves light's wave nature, showing how it dances through our world! ✨
• Watch as light bends in style, changing speed but not frequency when it meets new media! 🙌
• A lens isn’t just for glasses—it's a genius at bending light to form images! 🔍
• Mirror, mirror on the wall, teaches you about reflection and how light can create magical images! 🪞

Key Takeaways

• Understanding the electromagnetic spectrum is key to mastering optics. It spans from radio waves to gamma rays, with visible light falling somewhere in the middle 🌈.
• Light behaves as both a wave and a particle, showcasing its dual nature. This duality is evident in phenomena like polarization and diffraction 🤓.
• Polarization is a unique wave phenomenon where light waves oscillate in a particular direction, which can be manipulated using filters 😎.
• The index of refraction of a material tells us how much light will bend when entering a new medium, affecting its speed but not its frequency 🌊.
• Lenses and mirrors utilize the principles of refraction and reflection to form images, crucial for understanding optical instruments 🔍.

Overview

Welcome to day six of the AP Physics 2 live review, where we dive into the fascinating realm of optics! Optics is all about light—its behavior, interaction with different media, and how it forms the images we see. Today, our journey will take us through a host of intriguing concepts from the very basics of the electromagnetic spectrum to the intricate play of light waves in various scenarios.

We kick things off by unraveling the electromagnetic spectrum, discussing how different types of waves interact with matter, and then move into physical optics by exploring wave phenomena like polarization and refraction. Polarization, a vital concept, demonstrates the wave nature of light, while refraction sheds light on how it bends when transitioning between different media.

In our final segment, we delve into the world of lenses and mirrors. These everyday objects aren't just random accessories; they're pivotal in how we perceive the world. We explore how lenses focus light to form images and how mirrors reflect light, offering a hands-on understanding of geometric optics. This deep dive not only prepares students for exams but also enriches their appreciation of physics in daily life.

Chapters

• 00:00 - 03:00: Introduction and Overview The chapter introduces the topic of optics, transitioning from prior topics such as fluids, thermodynamics, electricity, and magnetism. It is part of a larger AP Daily Live Review series. The narrator expresses enthusiasm about the material to be covered, encouraging students who have already mastered other physics topics.
• 03:00 - 09:00: Electromagnetic Spectrum and Light Properties This chapter focuses on the electromagnetic spectrum and light properties. It marks the continuation of the course where the previous day was centered around magnetism. Today's discussions involve exploring light and optics, setting the stage for tomorrow's topic on modern physics. The instructor also prepares students for an upcoming session with Mr. Strattoman, who will guide them through exam formats and preparation strategies based on his proven teaching techniques. The instructor also emphasizes Mr. Strattoman’s high standing and his efficacy in exam strategy.
• 09:00 - 12:00: Introduction to Polarization The chapter titled 'Introduction to Polarization' discusses the expertise and involvement of a high school teacher, Mr. Strauderman, who is notably connected with the AP Physics 2 Development Committee. The chapter then introduces the session's agenda, which includes a discussion on physical and geometric optics. It promises a segment on the electromagnetic spectrum, specifically addressing the concepts of frequency, velocity, and wavelength.
• 12:00 - 17:00: Refraction and Snell's Law This chapter explores various concepts related to wave physics, beginning with polarization, continuing with refraction and diffraction, and their differences. The discussion extends to thin films, thin coatings, lenses, and mirrors. The aim is to clarify these concepts, and the speaker offers support via a link for further questions or clarifications.
• 17:00 - 27:00: Diffraction and Interference The chapter begins with a preamble about audience interaction and upcoming content, indicating a discussion around an electromagnetic spectrum in future videos. The narrator then mentions feedback from the audience and announces the inclusion of a homework assignment in the video. The chapter focuses on introducing and covering basic concepts related to the electromagnetic spectrum, which includes all types and frequency ranges of waves from radio waves.
• 27:00 - 33:00: Thin Films and Phase Shift The chapter discusses the electromagnetic spectrum, emphasizing gamma radiation as a key focus point. It explores the order of the spectrum from low to high frequency, detailing each type from radio waves to gamma rays, highlighting their wavelength and energy differences. Visible light is noted as distinct due to its clearly defined frequency range.
• 33:00 - 41:00: Lenses and Ray Diagrams In this chapter titled 'Lenses and Ray Diagrams,' the discussion centers around wavelengths, particularly those of visible light, which range from about 400 nanometers to about 700 nanometers. The naming of these wavelengths is described as arbitrary, relying on their interactions with the universe. However, the principal focus is on the interaction of visible light with human beings. An equation is introduced that connects the wavelength of light to its velocity and frequency, involving 'v' as the speed of light, along with its frequency.
• 41:00 - 43:00: Mirrors and Ray Tracing The chapter begins with an emphasis on the important relationship between velocity, frequency, and wavelength. It is crucial for understanding the subject matter, and learners are encouraged to memorize the formula and its possible algebraic rearrangements. The formula is presented as velocity equals frequency times wavelength, while its rearrangements are given as frequency equals velocity divided by wavelength, highlighting their importance for the upcoming discussions on mirrors and ray tracing.
• 43:00 - 46:00: Homework and Conclusion The chapter discusses the wave nature of light and mentions that in the upcoming discussion, evidence for the particle nature of light will be addressed. Light is described as having a dual nature, exhibiting characteristics of both waves and particles. The chapter highlights properties such as wavelength and frequency as related to light's wave nature and introduces the concept of light polarization.

2021 Live Review 6 | AP Physics 2 | Geometric and Physical Optics Transcription

• 00:00 - 00:30 okay so you have floated through fluids you are a hot shot at thermo you feel positive about electricity and you give magnetism one big thumb up welcome to day six where we're going to focus on optics welcome everybody to ap daily live review today oh we have got plenty of stuff to learn
• 00:30 - 01:00 just to catch you up with where we are in the course you've worked through all of this material yesterday we worked on magnetism today it's all about light and optics tomorrow will be my last day with you as we discuss modern physics but i need to get you ready for thursday's session with mr strattoman he's going to be covering the exam format and exam preparation strategies the same fantastic material that he's been doing with his students he's going to share with you remember mr stroderman is the high
• 01:00 - 01:30 school co-chair of the ap physics 2 development committee there is probably no high school teacher in this country more plugged in than mr strauderman so you do not want to miss thursday's session so hope you catch that i know i'll be watching today the plan is pretty straightforward we'll be talking about first physical optics and then geometric optics so what's up with the electromagnetic spectrum we'll discuss a little bit about the frequency velocity and wavelength of
• 01:30 - 02:00 waves how polarization works what refraction and diffraction are how they're different even though the names are kind of similar and then we'll talk about thin films and thin coatings and then we'll talk a little bit about lenses and mirrors so before we get started i want to make sure that you have this link if you type this if you scan it if you click the link in the video description it'll send a message to me and mr strattoman if anything we talk about doesn't click or if you need some more
• 02:00 - 02:30 clarification just send a message we'll discuss it in an upcoming video and i want to thank you for the feedback so far i was very surprised that so many of you really want a homework assignment so there is a quick one in this video all right so let's begin by discussing the electromagnetic spectrum the electromagnetic spectrum encompasses all types and frequency ranges of waves from radio waves all the way to
• 02:30 - 03:00 gamma radiation now you may recall from your ap daily videos that the order is radio waves microwaves infrared light visible light ultraviolet x-rays and gamma uh that's from low frequency to high frequency from from long wavelength to short wavelength from low energy to high energy now of these visible light is the only one that gets uh that has a really well defined frequency range visible light ranges from on the long
• 03:00 - 03:30 wavelength and about 700 nanometers and the short wavelength and about 400 nanometers the other namings are kind of arbitrary based on how they interact with the universe but visible light is all about how it interacts with us now this equation relates the wavelength of light to velocity and frequency so up there is v the speed of light that's its frequency and that's its
• 03:30 - 04:00 wavelength this is a really important relationship we'll be discussing this a lot today if you feel that you have a great handle on this fantastic if not this is one you're going to want to write down in all of its possible algebraic rearrangements velocity equals frequency times wavelength that would be a good one to write down frequency equals velocity over wavelength another good one to write down okay so
• 04:00 - 04:30 it is important to note we are discussing uh some evidence for the wave nature of light tomorrow we'll be talking about uh two pieces of evidence for the particle nature of light we say that light has a dual nature it does some waveish stuff and it does some particley stuff so properties like wavelength and frequency pertain to light's wave nature and the fact that light can be what's called polarized
• 04:30 - 05:00 is one piece of the evidence of flight's way of nature so to discuss polarization i figured we could start with a great example from our good friends over at oh physics their interactive physics simulations provide a lot of opportunities to see phenomenon that you couldn't really otherwise see so what i have here is their polarization simulation and here's what's going on just to sum it up for you over here on
• 05:00 - 05:30 the left side we have what's called unpolarized light light that is oscillating in every possible plane of oscillation but then that unpolarized light passes through what's called a polarizing filter that only lets waves of one particular orientation pass so this opening lets us know that only vertically oriented waves pass through so this light is unpolarized
• 05:30 - 06:00 oscillating in every direction this light is what we call polarized it's only oscillating in the vertical direction you'll notice that this vertically polarized light right there is able to get through this vertically oriented polarizing filter it gets through and the wave is undisturbed if however we were to rotate the filter you'll notice that the light coming through the polarizing filter well first off it's not as bright it has
• 06:00 - 06:30 a lower amplitude there's less of it but also that this light is now also polarized in a different direction and if i were to turn this polarizing filter not vertical not halfway but all the way flat look at that vertically polarized light here does not get through a horizontal polarizing filter evidence that light is polarized it can be stopped by two polarizing filters that are
• 06:30 - 07:00 offset by 90 degrees okay we're going to head back to our presentation though thank you oh physics that's not the last we'll see of it and again the vertical polarizing fill made vertically polarized light and the angled polarizing filter made dimmer light polarized at that new angle the horizontally polarized filter did not transmit any light so here we go you're going to want to pause on this because this section is quick
• 07:00 - 07:30 two lights a and b are viewed through a polarizing filter students estimate the perceived brightness of each source compared to its unfiltered brightness which light if either is polarized so look down column a at the brightnesses of bulb a look down column b at the brightnesses of bulb b which of those two light sources gives evidence of being polarized
• 07:30 - 08:00 that's right everybody i'm sure that you said it was b we can tell that light source b is polarized because its perceived brightness depends on the angle of the filter now if you've still got questions about the polarization of light mr blazo does an amazing job in unit 6 in his ap daily video about polarization okay time for a quick look at refraction what
• 08:00 - 08:30 it is and what it's not when light passes from one material into another material some of that light is reflected and some of it enters the second material the transmitted light well it might have a different speed in that new material but it'll always have the same frequency so the new speed depends on something called the index of refraction of the new material so let's quickly define that term this index of refraction is the ratio
• 08:30 - 09:00 of the speed of light in a vacuum that's its fastest speed 3 times 10 to the 8th meters per second divided by light's speed in that material we call that ratio n the index of refraction but n's kind of a weird number n gets higher the slower the light is notice the smaller number we put right there in the denominator the smaller value we have for v the bigger value we're going to have for
• 09:00 - 09:30 n so if the velocity of light in a material is really slow that material has a very high value for the index of refraction all right so a great example of the index of refraction comes from a phet simulation you may have seen this in class you may even have done this in class let's take a look at bending light from fet good friends at the university of chicago here we go
• 09:30 - 10:00 here is a light source i have it set as a laser right now it can switch from rays to waves and that light source as you see it is uh traveling through this material that's air into this material that's water you'll see air has an index of a fraction of 1.000 and yeah there are some non-zero numbers after that but basically the index of a fraction for air is 1. the index of a fraction for water and you certainly don't have to remember this is about 1.3 or so
• 10:00 - 10:30 well let's look at what happens as we change the angle at which the light is striking this air water surface now when i say the angle i'm referring to the angle measured off of a normal line a perpendicular line because normal means perpendicular so it's this angle that we'll be looking at and this angle that we'll be looking at now through the fantastic power of this simulation i can turn on the angle measurements and what you can see is that when light passes from air
• 10:30 - 11:00 into water even though the angle of incidence this is like theta with a little i the incident angle is 46 and a half degrees the refracted angle theta r is 33.0 degrees so it's a smaller angle in fact no matter how i angle this no matter what i do theta 2 the angle that light makes with the
• 11:00 - 11:30 normal line in the water is always going to be smaller than theta 1. no matter how i arrange that and what we can see is that as the index of refraction increases if i were to instead be shining this laser at the same angle through glass look at what's happening look what's happening that angle is getting smaller so the amount of bending depends on the index of refraction
• 11:30 - 12:00 so we're going to switch to the wave nature of light here for just a sec we're going to discuss this light as a wave you notice i have it set on slow motion because wow if this were full speed it would be hard to measure one of the things that i want to point out and it takes a moment to get this right now this little sensor just tells us the intensity of light as a function of time but we can use it as an indication of uh to help us compare the frequencies check that out the light here
• 12:00 - 12:30 and the light here have the same frequency even though they have different speeds the light up top here is moving at well c the speed of light in a vacuum that 3 times 10 to the 8th meters per second that's basically the speed of light in air but the speed of light in water i wonder what it is well the index of refraction of water is 1.333 so you should be able to calculate
• 12:30 - 13:00 the speed in water is going to be only three quarters of its value in air so even though there's a speed difference there is clearly not a frequency difference so let's look at what is different the wavelength of light in air i don't know if you can see this on the animation on your screen is bigger than its wavelength here in the water in order for the frequency to remain the same for the velocity to change the result is
• 13:00 - 13:30 that the wavelength decreases as the velocity decreases okay so we have an idea about the relationship between the index of refraction and the amount of bending we have an idea about the index of refraction and the new wavelength let's take a look back at uh some of the calculations for this so here are two media and there's some light passing through their boundary if you are looking for a great screen shot
• 13:30 - 14:00 just to grab the definitions here that's the angle of incidence the angle of refraction this material has index of a fraction and one this material has index of refraction n2 and light bends towards the normal when n2 is greater than n1 this relationship is called snell's law n1 sine theta 1 equals n2 sine theta 2. oh it looks like i have an i in there that ought to be a 1 n1 sine theta 1
• 14:00 - 14:30 equals n2 sine theta 2 where theta 1 is the angle of incidence and theta 2 is the angle of refraction if however n2 were less than n1 check this out light would be bending away from the normal and there's one more relationship that i'd like to show you we're going to click back to that simulation and i'm going to switch these media we're going to go from glass into air so there it is now
• 14:30 - 15:00 at this angle it totally follows snell's law n1 sine of 18 equals n2 sine 27.7 degrees all right great totally calculable and you'll notice the refracted angle is greater than the incident angle because the index of a fraction for air is less than the index of a fraction for glass but notice as i increase the incident angle look at that refracted angle this is
• 15:00 - 15:30 approaching 90 degrees and something real funny happens at 90 degrees let's see if we can get oh look at that look at that instead of refracting at 90 degrees the light is internally reflected back into the glass this is following the law of reflection sure it's doing what a mirror would do the angle of incidence equals the angle of reflection
• 15:30 - 16:00 but there is no transmitted beam there is no light that leaves the glass and enters the air let's have a quick chat about that if theta 1 the incident angle is at least big enough to cause theta 2 to be 90 degrees there is no refracted beam so if n1 sine theta 1 equals n2 sine 90 there's no refracted beam the light just reflects
• 16:00 - 16:30 we call this angle that angle 1 there that theta 1 there on the bottom we call that the critical angle the angle at which total internal reflection occurs at that theta 1 and greater theta 1's there is total internal reflection okay so you may have done this as well this activity
• 16:30 - 17:00 let me switch this back to air and switch this back to let's say let's say glass now you may have done this with actually putting a protractor right at that interface right there and i'm going to take this angle measurement off because that gets a little bit sloppy and you may have measured incident angles and refracted angles through a whole bunch of different angles if you did this activity you may have done it in order to figure out the index of refraction of
• 17:00 - 17:30 some unknown material maybe you even used this one if you haven't i would challenge you do it find the index of refraction of mystery material n we have all of these incident and refracted angles to choose from all right i'm gonna head back to the presentation here now if you gathered a whole bunch of measurements from that and you were to graph them if you were to graph uh theta one versus
• 17:30 - 18:00 theta 2 you wouldn't have a great relationship but if you were to graph the sine of theta 1 versus the sine of theta 2 you would have a relationship that makes a linear plot like this and being a smart ap physics student you i know i know you would would draw a line of best fit and then say cool now i see the pattern if you if you've done this if you've encountered this you probably found the slope of the line using the line and not the data and
• 18:00 - 18:30 figured out that the slope is the ratio of n1 to n2 in this relationship n1 sine theta 1 equals n2 sine theta 2. so the slope here is n1 divided by n2 if you encounter this in your in your ap physics 2 maybe on your final exam or on the ap physics 2 exam this would make a great lab question it involves linearization involves
• 18:30 - 19:00 taking careful data so i hope you got a chance to do that one if not you you probably should try out the interactive simulation right here phet.colorado.edu all right now it's time to talk about diffraction another wave property of light diffraction occurs when a wave passes through a small opening and spreads out the light makes a pattern of bright and dark spots you may have seen this before the pattern tells us about
• 19:00 - 19:30 the wave and about the opening but of course it only works if the the wavelength of the wave is roughly the same size of the opening we say about by a factor of 10. diffraction demonstrates light's wave nature the wave passes through multiple small openings and interferes with itself yeah waves do that they can interfere with themselves exactly the way that bowling balls can't because interference is a property that is unique to waves which we can see here
• 19:30 - 20:00 at this diffraction demonstration at the physics aviary so i'm going to click to that we'll take a look at this demonstration right here we have two they're a little bit hard to see in your view because i've got a copy of the student view right here there are two closely spaced openings in this wall we're going to send a wave through that wall and look at what happens on the other side now as you see each of these openings
• 20:00 - 20:30 is acting like its own wave source i'll pause this in just a moment and what you see is that along some of these lines um the wave crests add up so about there i see some wave crests adding up along this line i see wave crests adding up and along this line i see wave crests adding up crests are meeting crests and troughs are meeting troughs that's what we call constructive interference but along these red lines somewhere in the middle
• 20:30 - 21:00 you'll notice that crests are meeting troughs and crests are meeting troughs we call that destructive interference the amplitude of the wave is adding up to zero that is a behavior that is unique to waves interference is unique to waves let me switch right back to here all right so i tried this demonstration twice and i did trace all the waves i traced the areas of constructive interference and what
• 21:00 - 21:30 you'll notice here is and i'll label these a little bit differently i'm going to circle these in red i'm going to circle these two openings there's one right there circled in red there's another one right there the red circle on it those are the two openings you'll notice that those two slits are really closely spaced and that the maxima are spread far apart but here i repeated this mission you'll notice that these slits are far apart
• 21:30 - 22:00 and the maxima are closely spaced so if the slits are close the maxima are far and if the slits are far apart the maxima are closed together now you may have done this lab virtually you may also have done it with a diffraction grating and a laser i'm going to switch back to a demonstration from o physics now and we'll take a look at what happens when we shine a laser through what's called a diffraction grating
• 22:00 - 22:30 a diffraction grating is just a lot of really closely spaced openings makes the laser do that in all of these situations i'd like you to focus on two things one just looking at you know the spread of the light as it goes through the diffraction grating but also keep your eyes up here up here on this ruler this is a zoomed in picture of what's on the screen this is what you'd be measuring if you
• 22:30 - 23:00 were doing this lab in class you'd be looking at the spacing of the dots now i think probably intuitively i'll put a green dot up on top where these green dots are oh that's a square that'll work so those two green squares that i just drew represent where the dots are now but if i were to bring this screen closer maybe half as far away you'll notice that those dots are about half as spread out now so as i move the screen where i'm viewing it it changes how spread apart the dots are
• 23:00 - 23:30 i think that relationship is kind of intuitive but this next one i find pretty amazing i want to reduce the number of lines per millimeter the minimum number of lines per millimeter i can have is 200 lines per millimeter so that means they're not very closely spaced they're kind of spread apart there's only 200 of them per millimeter as i increase the number of lines per millimeter though see if i can park this at around 400 so
• 23:30 - 24:00 it's twice as many look at that with twice as many lines per millimeter those dots are twice as far apart and i'll increase it even more to 500 lines per millimeter so notice the more closely i space those lines the farther apart the diffraction pattern is the farther apart these dots are up here those dots are really far apart and if i space out the diffraction grating those dots get way closer together
• 24:00 - 24:30 okay we also need to understand as smart ap physics folks the effect of the wavelength of the laser on the spread of the diffraction pattern so here we have a green laser about 532 nanometers if i decrease the energy if i increase the wavelength if i decrease the frequency so it's red light you'll notice that red light spreads out a lot and now i'm increasing the frequency
• 24:30 - 25:00 decreasing the wavelength move through blue all the way to violet that violet laser spreads out way less than the red laser did so the amount of spread depends on a couple of different factors so on the left here you see a diffraction grating of 200 lines per millimeter in the middle 350 lines per millimeter on the right 500 lines per millimeter that's a relationship you're going to need to be able to discuss
• 25:00 - 25:30 on the ap physics exam another example uh we're diffracting 400 nanometer light on the left and 700 nanometer light all the way on the right look at the difference in that pattern so if you want to see why the bright spots appear where they do and how that relates to the path difference in that light and how if the path difference is just right we get constructive interference and if it's just right we get destructive
• 25:30 - 26:00 interference ms mosley does a great job of explaining that in a very visual way daily video 3 and topic 6.6 but if you didn't get to do this lab in class well you can do it right along with mr blazo that's topic 6.6 video 4. okay so diffraction works like this one light wave goes through two openings and interferes with itself the light has wavelength lambda and
• 26:00 - 26:30 those maxima disperse at angles of theta d sine theta equals m lambda where d is the distance between those slits and m is uh one of those variables counting to the mth bright dot so like m equals one right here at the first bright dot or m equals two right here at the second bright dot if that angle is really small if that angle is less than a few degrees
• 26:30 - 27:00 you can use what's called the small angle approximation and instead of using d sine theta equals m lambda use x equals m lambda l over d where x is the distance between those m's all right take home lesson is that a narrower slit spacing causes a wider spread in the dots or a wider x longer wavelengths cause a wider theta or a wider x now here is a question you are unlikely to
• 27:00 - 27:30 see on the ap physics exam where they just give you a bunch of data and ask you to find the distance between some dots you can do it thanks to those of you in the feedback who said that you liked the color coding this is for you fully color coded and there's a solution and it's calculable but it's not an interesting question this doesn't ask if you understand physics it asks if you can push buttons this question i think asks if you can understand physics
• 27:30 - 28:00 students are shining a laser through through a hole onto a wall and they see this pattern how could they increase the spacing between the bright dots how could they make those bright dots closer together well what if they used a violet laser a violet laser has a smaller wavelength than the green laser so it would have a smaller diffraction pattern what if they used a wider hole well a wider hole would be like a wider
• 28:00 - 28:30 d so that would cause a narrower diffraction pattern but if they back away from the wall they're going to increase the spread and that that's a happy question that is evidence that you have done some physics now you're thinking physics now you're thinking like a physics student all right one more wave property to talk about here and that's what's going on with thin films the the shimmering swirly colors that you see when you look at a soap bubble that's due to the
• 28:30 - 29:00 interference of waves the light that you see is interference between two different waves first off one wave bounces off the outer surface of that soap bubble and then another wave bounces off the inner surface of the soap bubble and both waves get to your eyeball interacting with each other wow so let's take a look at what's going on there because there's one weird thing you may recall from class called a phase shift we need to spend a moment on
• 29:00 - 29:30 that when when light traveling through air reflects off of water there's what's called a 180 degree phase shift you might notice that we've got crests and troughs and crests troughs and crests and you'd expect a trough next wouldn't you it's bouncing off as a crest trough crest trough well that's not in order so that is a 180 degree phase shift and notice air has a lower
• 29:30 - 30:00 index of refraction than water when light bounces off a surface after when light traveling through one material bounces off the surface of another material if the second material has a higher index of refraction there's what's called a phase shift the wave essentially flips over but now we're going to draw that same light wave traveling through water and when it travels through water it bounces off air there is no phase shift trough crest trough crest trough crest etc
• 30:00 - 30:30 if that all came at you too fast if n1 is less than n2 there's a phase shift if n1 is greater than n2 there's no phase shift okay so to make those uh those swirls seen in a soap bubble several things happen to that incoming light first some of it reflects off the top and it changes phase second some of that light refracts into the material and it follows snell's
• 30:30 - 31:00 law it has a slower velocity and a shorter wavelength and some of that light just refracts out the other side following snell's law but some of that light that was inside the soap reflects off the back of the soap film the inner layer undergoes no phase change refracts back out and you see that reflection so i've tried to draw this process we're going to look at a quick example of light doing this through this material i've drawn a thin
• 31:00 - 31:30 film it's this orange to maroon goop there and we're going to shine green light into it and like i said some of that green light is just going to bounce off it's going to follow the law of reflection but some of that light will be refracted into the material now some of that refracted light is going to refract out of the material and just go through but i want you to look right here i'm going to erase that circle but put
• 31:30 - 32:00 your eyes right there there's going to be a reflected ray there it is that's the inner reflected ray follow that ray as it leaves the material and refracts out changing angle again all right i'm going to dim the other arrays and we're just going to look at those two rays that come out that's these two rays right here those are the ones that we're going to look at with our eyes and the way that i've drawn this and the way that i've spaced out this very specific example is that i have crests meeting
• 32:00 - 32:30 troughs and that means because crests are meeting troughs there's going to be what's called destructive interference if crests were meeting crests there would be constructive interference we would see bright green light here because the crests are meeting the troughs we aren't going to see the color green in this reflection at all we might see blues and purples and reds but we're not going to see greens as you are solving problems like this or even thinking about situations like this
• 32:30 - 33:00 here's what you ought to be thinking about is there a phase change at that top surface is there a phase change at the bottom surface what's the path difference and if the path difference and whether or not there are phase changes line up so that crests are meeting crests well you're going to see that color amplified in the reflection but if the path difference in the phase changes are
• 33:00 - 33:30 such that crests are meeting troughs you aren't going to see that color in the reflection now again this is a big unit it's coming at you really quick if you know you've got a bunch of questions about thin film interference check out daily video 5 in topic 6.6 miss mosley does a great job with this okay time to talk about lenses we're through talking about uh what are called physical optics now we're talking about geometric optics that's lenses and mirrors light
• 33:30 - 34:00 refracts once as it enters a lens and refracts again as it exits the lens oops pardon me the result of these two refractions will be different depending on the shape of the lens diverging lenses cause light to spread out converging lenses cause light to come together and lenses can form what are called images if the light rays cross before they get to your eyes you're looking what's called a real
• 34:00 - 34:30 image real images can be projected on a piece of paper or a screen if the light rays don't cross before they get to your eyes you're looking at what's called a virtual image concave lenses like this the ones that scoop in those are called diverging lenses because the refracted rays always diverge concave lenses only form virtual images so here's a bunch of parallel light rays
• 34:30 - 35:00 striking the surface of the lens and there are two refractions to look at refraction number one occurs as light enters the lens notice those light rays bend they traveled slower in the lens as the light rays leave the lens they refract again look at that that's the second refraction i was talking about now here's a convex lens convex lenses are also known as converging lenses because well passing light rays converge convex lenses are capable of forming real or virtual images
• 35:00 - 35:30 so here is light parallel light rays coming towards the lens refracting inside the lens and then refracting as they leave the lens again two refractions okay now i'm only giving a few examples on ray tracing there are more and better examples on ray tracing and i'll let you know where to get those it's going to be on ap daily isn't it so the rules for ray tracing and i'm sure
• 35:30 - 36:00 you've probably practiced a whole bunch of these are these uh raise parallel to the principal axis that's that dotted line refract away from the focus for a concave lens so here is a ray parallel to the principal axis and there it is refracting away from the focus that dotted line on the lower left side is just so you know where that ray came from rays through the center well they're unrefracted they leave at the same angle and that i know is an approximation for
• 36:00 - 36:30 thin lenses but in ap physics 2 it's a pretty good approximation so there it is a ray drawn through the center unrefracted for a convex lens raised parallel to the principal axis refract right through the focus there's the ray parallel to the principal axis there it is refracting through the focus and again raised through the center they leave at the same angle the place where these two actual rays actually cross
• 36:30 - 37:00 is where your eye if you were looking at this through this lens your eye would see the image there also it is projectible that is a projectable image so if you look through this lens you would see an upside down image that appeared to be there in space but if you were to put a screen or a piece of paper or an index card right there you would see a projected upside down image alrighty the equation that links all
• 37:00 - 37:30 this together is called the thin lens equation you'll find it right on the ap physics 2 equation sheet credited right there ap physics 2 equations the important things to know about lenses are here you should definitely take a screenshot of this right now it tells you what diverging lenses and converging lenses virtual images and real images are have you grabbed a screenshot i hope so
• 37:30 - 38:00 okay cool let's keep moving if you want more practice with ray tracing it's ap daily video 1 and topic 6.5 but if you want to solve more lens problems with calculations that's daily video 3. same topic however you're representing it either by doing a calculation or by drawing the diagram you're really doing the same thing light follows the law of refraction because light travels at different
• 38:00 - 38:30 speeds in different media and the result is that carefully constructed lenses are able to form images using refraction now i'm hoping that you've done a lab like this it's a pretty common optic setup that'll help you find the focal length of a lens by finding combinations of object distance and image distance that result in a clear image being formed in this lab students would attach a candle to one end of a ruler and a screen somewhere else and move that lens
• 38:30 - 39:00 between them until a really clear image of the candle is formed on the screen if you were to do this lab you would get a lot of data and the data that you get would trace out this pattern if you collected data on the object distance and the image distance it would get that pattern well clearly based on the equation that that we've seen one over so plus one over si equals one over f that suggests maybe the relationship has
• 39:00 - 39:30 to do with the inverses so maybe you have had a chance to linearize this data and made something like this graph from this graph we can figure out the focal length of that lens we can rearrange it like this so that one over s i is on the left side of our equation and if we just take a look at that equation we'll see that one over s i is our y axis variable the slope should be negative one the x-axis variable should be one over s
• 39:30 - 40:00 o and the y-intercept should be one over the focal length look at that that's the y-intercept that's 1 over the focal length i wonder what that x-intercept is [Music] you should try that out all right finally we're here to talk about mirrors now here are the rules for ray tracing in a mirror raised parallel to the principal axis reflect through the focus rays that travel
• 40:00 - 40:30 through the focus re reflect parallel to the principal axis that takes care of concave mirrors how about convex mirrors raise parallel to the principal axis reflect away from the focus check it out that ray follows that line away from the focus ray's heading for the focus reflect parallel to the principal axis so this ray was on its way to the focus and reflected parallel to the principal axis
• 40:30 - 41:00 now we're just going to look at these two rays if you were looking at the reflection from this mirror you would only see those two uh darker more opaque rays the two reflected rays and your eye and your brain team up to form an image if you look at a reflection of yourself maybe in the curved back of a spoon you see a little person that lives inside the spoon hello little person in
• 41:00 - 41:30 the spoon you see what's called a virtual image now actual light rays didn't go there but you traced those rays backwards to find that all right ray tracing gets a little bit weird when the object is closer than one focal length and that's a a little bit worth talking about here for a concave mirror raised parallel to the principal axis reflect through the focus and there is one more rule rays that strike the center well they
• 41:30 - 42:00 follow the law of reflection they reflect at the same angle and if our eye were over here here's me hello if i were looking at these reflected rays i would say oh clearly those rays come from over there so clearly there is a magnified image and upright magnified virtual image right there well that's a lot ray tracing is tough if you like practice ray tracing with mirrors join up with mr blazo in ap
• 42:00 - 42:30 video ap daily video 2 and topic 6.5 the mirror equation looks a lot like the lens equation 1 over si plus 1 over so equals 1 over f but there's a little bit more to know i i think i always find mirrors a little bit more confusing than lenses maybe that's just me but you're going to want a screenshot of all of this as well everything you need to know about convex mirrors and concave mirrors virtual images and real images formed by those mirrors virtual images are always
• 42:30 - 43:00 upright upright images are always virtual inverted images are always real real images are always inverted and all this works because light follows the law of reflection the angle of incidence equals the angle of reflection so as promised folks here is your homework we've got a group of students they want to project a photo of a tree they have a photo of a tree on their phone and they want to project it through a lens onto a wall in their classroom
• 43:00 - 43:30 the image they're hoping is going to be larger than the phone screen they want to make this thing a magnified image so that the rest of the students around them can see it and the tree in the picture on the wall should look upright so student a says let's hold the phone eight centimeters from the lens it'll act like a magnifying glass pardon me student b says let's hold the phone 25 centimeters away
• 43:30 - 44:00 we'll focus a real image on the wall pardon me and student c says that so grab a screenshot because here comes the question for each student which aspects of their idea if any are correct which aspects of their idea if any are incorrect oh now you're thinking i should have grabbed a
• 44:00 - 44:30 screenshot there it is get your screenshot alright so your answer should look something like this student a tell me what they said correct and tell me what they said that you disagree with all right here are the big take home messages what should you take away what are the important lessons well for physical optics energy and frequency and wavelength are related and you should know where they are on the em spectrum you should know that light moves at
• 44:30 - 45:00 different speeds in different media then a light rays frequency is the same in different media you should know that polarization is evidence of light's wave nature and that when light changes media and changes speed it bends when a light wave travels through a small opening it interferes with itself and it does diffraction and light reflecting off the inner and outer surface of a thin film interferes with itself those are wave properties of light
• 45:00 - 45:30 and then the key takeaways as we talked about geometric optics is that light follows the law of refraction when entering or leaving a lens and light follows the law of reflection when bouncing off a mirror both of which can form images that can be found using ray diagrams or equations okay i know that was a lot i tried to reference all of the ap daily videos just the best of the bunch that are gonna help you out the most uh but thank you very much for coming along for that
• 45:30 - 46:00 illuminating journey uh tomorrow will be my last day uh with ap daily live review with you but uh hey we'll be learning about modern physics that's gonna be awesome but don't worry the fun is far from over on thursday uh gentlemen and scholar mr other strattoman we'll be back to wrap up the whole course and send you off on your way to success so again thank you very much you brilliant ap physics students i look forward to joining you tomorrow stay awesome
• 46:00 - 46:30 you