LECTURE 13 REFRACTION. Instructor: Kazumi Tolich

Transcription

1 LECTURE 13 REFRACTION Instructor: Kazumi Tolich

2 Lecture 13 2 Reading chapter 26.5 Index of refraction Snell s law Total internal reflection Total polarization

3 Index of refraction 3 The speed of light in a transparent medium is less than the speed of light in vacuum. A transparent medium is characterized by an index of refraction, n, given by n = c v c: speed of light in vacuum v: speed of light in the medium

4 Snell s law/demo: 1 4 Light travels from one point to another along the path that takes least time. The angle of transmitted ray, θ 2, is called angle of refraction. The angles of incidence and refraction, and indices of refraction of two media are related by Snell s Law: n % sin θ % = n * sin θ * Lifeguard analog

5 Quiz: 1 5 Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster? A. medium 1 B. medium 2 C. both the same air air Medium 1 Medium 2

6 Quiz: 13-1 answer 6 Medium 1 The greater the difference in the speed of light between the two media, the greater the bending of the light rays. Follow-up: How does the speed of light in air compared to the speeds of light in medium 1 or medium 2? air air Medium 1 Medium 2

7 Quiz: 2 You are trying to catch fish by using a spear. You observe a large fish a couple of meters in front of you and a meter below the water surface. Assuming that the fish is stationary, in order to hit the fish with your spear you must aim A. directly at the point where you see the fish. B. slightly above the point where you see the fish. C. slightly below the point where you see the fish.

8 Quiz: 13-2 answer You must aim slightly below the point where you see the fish. For you to see the fish, the light must travel from the fish to your eyes. The ray is refracted at the water-air boundary. The angle of incidence is smaller than the angle of refraction since light travels faster in air than in water. Follow-up: To shoot a fish with a laser gun instead, should you aim directly at the image, slightly above, or slightly below?

9 Example 1 9 A 2.00 m long vertical pole extends from the bottom of a swimming pool to a point 50.0 cm above the water. Sunlight is incident at 55.0º above the horizon. What is the length of the shadow of the pole on the level bottom of the pool? The indices of refraction of air and water are 1.00 and 1.33, respectively.

10 Total internal reflection/demo: 2 10 When the incident light is in a medium with a higher index of refraction, the angle of refraction is greater than the incident angle. Total internal reflection occurs when the incident angle is greater than the critical angle, θ +, and no refracted ray exists. sin θ + = n * n %

11 Quiz: 3 11 In an optical fiber, what can be said about the index of refraction of the core, n +,-., versus the index of refraction of the cladding, n +/011234?

12 Quiz: 13-3 answer 12 n +/ < n +,-. In order for there to be total internal reflection light must travel in a higher index of refraction and incident on a boundary with medium of a lower index of refraction. Total internal reflection in fiber optic light pipes makes it possible to transport light and light-encoded signals over long distances without significant loss.

13 Quiz: 4 13 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

14 Quiz: 13-4 answer 14 No The rays that make smaller incident angles than the critical angle still make out of water and span the whole space above water.

15 Example: 2 15 A light bulb is set in the bottom of a 3.0 m deep swimming pool. What is the diameter of the pool s surface where the light can escape? h = 3.0 m

16 Total polarization 16 When light is incident at Brewster s angle, θ 6, the reflected and refracted beams are at right angles to one another, and the reflected light is totally polarized parallel to the reflecting surface. tan θ 6 = n * n %

17 Quiz: 5 17 What kind of Polaroid sunglasses would be useful in reducing the glare at sea? The stripes represent the directions of the transmission axes.

18 Quiz: 13-5 answer/demo: 3 18 A Since the glare is horizontally polarized, Polaroid sunglasses with vertical transmission axis will absorb the horizontally polarized light. Glasses C are useful for viewing 3D movies.

19 Example: 3 (Walker Ch ) 19 While studying physics at the library late one night, you noticed the image of the desk lamp reflected from the varnished tabletop. When you put on your Polaroid sunglasses to look cool (why not, right??), the reflected image disappears. If this occurs when the angle between the incident and reflected rays is 110, what is the index of refraction of the varnish?