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Physics 212 Lecture 25 Physics 212 Lecture 25 Physics 212 Lecture 25, Slide 1 Music Who is the Artist? A) The Band B) Bob Seger C) CCR D) Eagles E) Steve Miller Band We’ve had a few CCR requests Your Comments “Finally! Stuff I actually like and understand!” “Probably the easiest thing we will ever learn all year.” “I didn’t really understand the polarized portion, partially because the prelecture slide was 3 minutes of a non moving picture We’ll go over the key steps which surely cannot hold my attention when I’m on a computer. ” “I am confused about the polarization. Why is the horizontal polarization unchanged while the vertical polarization is decreased from incidence to reflected ray??” “I could use a little more discussion on the polarization due to reflection.” “I thought the prelecture was pretty straight forward (minus the slide about intensity - uuuhhh Checkpoints – coming up! a little more explanation por favor!), but then, the checkpoints were a bit trickier than I expected. Could we go over them please????” Hour Exam 3: 1 week from tomorrow (Wed. Apr. 25) • covers L19-26 inclusive (LC circuits to lenses) • Sign up for conflicts, etc. no later than Mon. Apr. 23 at 10:00 p.m. 05 Physics 212 Lecture 25, Slide 3 Let’s start with a summary: from n1 to n2 from n1 to n2 Physics 212 Lecture 25, Slide 4 The speed of light in a medium is slower than in empty space: since 0 < vmedium = c / nmedium Remember: is dielectric constant (capacitors) Physics 212 Lecture 25, Slide 5 A ray of light passes from air into water with an angle of incidence of 30 degrees. Checkpoint 1a Which of the following quantities does not change as the light enters the water? A. wavelength B. frequency C. speed of propagation “frequency depends on speed, which obviously changes” “frequency doesn’t depend on the medium. Frequency only depends on the source producing the wave” “Speed of light is always the same.” Physics 212 Lecture 25, Slide 6 A ray of light passes from air into water with an angle of incidence of 30 degrees. Checkpoint 1a Which of the following quantities does not change as the light enters the water? A. wavelength B. frequency C. speed of propagation What about the wave must be the same on either side ??? Observers in both media must agree on the frequency of vibration of the molecules Physics 212 Lecture 25, Slide 7 Reflection Physics 212 Lecture 25, Slide 8 Refraction: Snell’s Law D sin 2 Dsin 1 n2 sin 2 n1 sin 1 c / n2 c / n1 Physics 212 Lecture 25, Slide 9 Think of a day at the beach… What's the fastest path to the ball knowing you can run faster than you can swim? This one is better Not the quickest route… Physics 212 Lecture 25, Slide 10 A x1 l1 y1 Same Principle works for Light !! D y2 l2 x2 B l l x2 y2 x2 y2 Time from A to B : t 1 2 1 1 2 2 v1 v2 v1 v2 To find minimum time, dt x1 x2 dx2 differentiate t wrt x 1 dx 2 2 2 2 dx and set = 0 1 v1 x1 y1 v2 x2 y2 1 dx2 How is x2 related to x1? x2 D x1 1 dx1 x x sin sin Setting 1 2 0 1 2 v = c/n n1 sin 1 n2 sin 2 dt/dx1 = 0 v1l1 v2l2 v1 v2 Physics 212 Lecture 25, Slide 11 The path of light is bent as it passes from medium 1 to medium 2. Checkpoint 2a Compare the indices of refraction in the two media. A. n1 > n2 B. n1 = n2 C. n1 < n2 “by snell’s law, n1 has to be bigger than n2 to make up for smaller sin(theta1) ” “They are the same angles. ” “if theta is smaller, n must be smaller” Physics 212 Lecture 25, Slide 12 The path of light is bent as it passes from medium 1 to medium 2. Checkpoint 2a Compare the indices of refraction in the two media. A. n1 > n2 B. n1 = n2 C. n1 < n2 Snell’s Law: n1sin1 = n2sin2 n decreases fl increases Physics 212 Lecture 25, Slide 13 Total Internal Reflection o NOTE: n1 > n2 implies 2 > 1 BUT: 2 has max value = 90 !! 1 > c Total Internal Reflection Physics 212 Lecture 25, Slide 14 A light ray travels in a medium with n1 and completely reflects from the surface of a medium n2. Checkpoint 2b The critical angle depends on: A. n1 only B. n2 only C. both n1 and n2 “n1 determines n2 which determines the critical angle” “Critical angle depends on the incidence angle inside the medium, so only n2 is required. ” “n1 must be greater than n2” Physics 212 Lecture 25, Slide 15 A light ray travels in a medium with n1 and completely reflects from the surface of a medium n2. Checkpoint 2b The critical angle depends on: A. n1 only B. n2 only C. both n1 and n2 c clearly depends on both n2 and n1 Physics 212 Lecture 25, Slide 16 Intensity Anything looks like a mirror If two materials have the same n if light is just glancing off it. then its hard to tell them apart. Physics 212 Lecture 25, Slide 17 Polarization 56.3o 1 2 90 sin 2 sin( 90 1) cos1 n2 Snell’s Law: n2 sin2 n2 cos1 n1 sin1 tan1 n1 Physics 212 Lecture 25, Slide 18 A ray of light passes from air into water with an angle of incidence of 30 degrees. Checkpoint 1b Some of the light also reflects off the surface of the water. If the incident light is initially unpolarized, the reflected light will be A. unpolarized B. somewhat horizontally polarized C. somewhat vertically polarized “reflection doesn’t polarize previously unpolarized light.” “The vertical component of the E field gets smaller as the ray approaches Brewster&#39;s angle.” “Only vertically polarized light reflects” Physics 212 Lecture 25, Slide 19 A ray of light passes from air into water with an angle of incidence of 30 degrees. Checkpoint 1b Some of the light also reflects off the surface of the water. If the incident light is initially unpolarized, the reflected light will be A. unpolarized B. somewhat horizontally polarized C. somewhat vertically polarized o = horizontal Physics 212 Lecture 25, Slide 20 A ball sits in the bottom of an otherwise empty tub at the front of the room. Suppose N people sit high enough to see the ball (N = ). Physics 212 Lecture 25, Slide 21 A ball sits in the bottom of an otherwise empty tub at the front of the room. Suppose N people sit high enough to see the ball (N = ). Suppose I fill the tub with water but the ball doesn’t move. Will more or less people see the ball? A) More people will see the ball B) Same # will see the ball C) Less people will see the ball ?? A w Snell’s Law: ray bent away from normal going from water to air Physics 212 Lecture 25, Slide 22 A light is shining at the bottom of a swimming pool (shown in yellow in the figure). A person is standing at the edge of the pool. Checkpoint 3 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection at the water-air surface? A. yes B. no “light could all be relected and stay in the water so he can’t see it ” “it depends on the position of the light. if it’s too close to the person, they won&#39;t see it.” “Light comes out at many angles, so while some may be interneally reflected, all cant since they are at differnet angles” “There would be a point where some light would be totally reflected but that would only be for a point, not for the whole light.” Physics 212 Lecture 25, Slide 23 A light is shining at the bottom of a swimming pool (shown in yellow in the figure). A person is standing at the edge of the pool. Checkpoint 3 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection at the water-air surface? A. yes B. no The light would go out in all directions, so only some of it would be internally reflected. The person would see the light that escaped after being refracted. DRAW SOME RAYS Physics 212 Lecture 25, Slide 24 Example: Refraction at water/air interface • Diver’s illusion 97º Diver sees all of horizon refracted into a 97°cone nn1 sin aa sin90 air 90 water water 48.5 nnww1.33 Physics 212 Lecture 25, Slide 25 Exercise 45o A meter stick lies at the bottom of a n = 1.33 rectangular water tank of height 50cm. water You look into the tank at an angle of 45o relative to vertical along a line that 50 cm skims the top edge of the tank. What is the smallest number on the ruler that you can see? 020 406080100 Conceptual Analysis: - Light is refracted at the surface of the water Strategy: - Determine the angle of refraction in the water and extrapolate this to the bottom of the tank. Physics 212 Lecture 25, Slide 26 Exercise 45o A meter stick lies at the bottom of a n = 1.33 rectangular water tank of height 50cm. water You look into the tank at an angle of 45o relative to vertical along a line that 50 cm skims the top edge of the tank.
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