Refraction Ch. 29 in your text book

Transcription

1 Refraction Ch. 29 in your text book Objectives Students will be able to: 1) Identify incident and refracted angles 2) Explain what the index of refraction tells about a material 3) Calculate the index of refraction based on the refracted angle

2 Refraction When light enters a material, it changes speed. The frequency of the light stays the same. When it changes speed, it also changes the angle at which it is traveling. Check for understanding If the speed changes and the frequency doesn t, what about the wavelength? Remember, v = λf Example: Say you were pulling a wagon on the grass in a park. If one of the wheels were to hit a soggy patch of grass, how would the motion of the wagon change?

3 Refraction Refraction The bending of a wave when it crosses the boundary between two different materials The part of the light wave in the new material travels at a different speed than the part of the wave that is not in the new material How much it bends depends on the index of refraction.

4 Refraction Index of refraction The ratio of the speed of light in a vacuum to the speed of light in a medium - the ratio will always be greater than 1 - no units n = c v n index of refraction c speed of light in vacuum v speed of light in material Check for understanding: Material A has an index of refraction of 1.5 and material B has an index of refraction of In which material does light travel faster?

5 Refraction Examples of refractive indices for different materials. Check for understanding Based on the table to the left, how fast does light travel through water? n = c v 1.33 = 3 x 108 v v = 2.26 x 10 8 m/s

6 Refraction Check for understanding What is the index of refraction for a plastic if light travels 2.68 x 10 8 m/s in the plastic? n = c v n = 3 x x 10 8 n = 1.12

7 Refraction Incident angle the angle between the normal and the incident ray of light Refracted angle the angle between the normal and the refracted ray of light Incident angle Incident ray θ θ Refracted angle Refracted ray Notice that the incident ray and the refracted ray are on opposite sides of the normal line

8 Refraction Refraction Lab

9 Snell s Law How do we know which way the light will bend? The direction the light bends depends on the index of the materials. If the light goes from a material with a smaller index to a larger index, the light will bend closer to the normal. n 1 = 1 n 2 = 1.5

10 Snell s Law How do we know which way the light will bend? If the light goes from a material with a larger index to a smaller index, the light will bend away from the normal. n 1 = 1.5 n 2 = 1

11 The light from the fish bends when it leaves the water and that it makes us think the fish is in a different location than where it actually is. Snell s Law

12 Snell s Law Draw which way the light will bend when it enters the green? n 1 = 1 n 2 = 1.1

13 Snell s Law Draw which way the light will bend when it enters the green? n 1 = 1 n 2 = 1.5

14 Snell s Law Draw which way the light will bend when it enters the green? n 1 = 2 n 2 = 1.5

15 Snell s Law Draw which way the light will bend when it enters the triangle? n 1 = 1 n 2 = 1.5 How will the light bend when it leaves the triangle?

16 Snell s Law When light refracts, it bends. Each wavelength bends at a different angle. When light has to refract a couple times, the differences in the angles becomes more obvious and we see all the colors.

17 Snell s Law Rainbows are formed by refracting light. Light refracts in the raindrops, reflects once, and then refracts again when it comes out, but a couple things have to line up just right for you to see it. Sometimes the light will reflect an extra time in the raindrop and you will get a double rainbow. The two rainbows will always be reflections of each other. Violet is usually hardest to see in the second rainbow.

18 Snell s Law n 1 sin(θ 1 )= n 2 sin(θ 2 ) n 1 index of refraction for one material n 2 index of refraction for the other material θ 1 angle for one material θ 2 angle for the other material θ 1 θ 2 n 1 n 2 Mathematically, it doesn t matter which material is 1 and which is 2. It does matter that the corresponding angle and the index of refraction stay together.

19 Snell s Law Light from the sun passes from air to a piece of plastic. The light hits the plastic at an angle of 50 to the normal. Inside the plastic, the light makes an angle of 40 to the normal. The index of refraction for air is 1. What is the index of refraction for the plastic? Hint: make sure your calculator is in degree mode 50 n 1 = 1 40 n 2 =? n 1 sin(θ 1 )= n 2 sin(θ 2 ) (1)sin(50)= n 2 sin(40) n 2 = 1.19

20 Snell s Law Light from the sun passes from air to a piece of plastic. The light hits the plastic at an angle of 50 to the normal. The index of refraction for air is 1 and the index of refraction for the plastic is 1.5. What is the angle of refraction inside the plastic? Hint: make sure your calculator is in degree mode 50 θ 2 n 1 = 1 n 2 = 1.5 n 1 sin(θ 1 )= n 2 sin(θ 2 ) (1)sin(50)= (1.5)sin(θ 2 ) sin(θ 2 ) =.51 θ 2 = sin -1 (.51) θ 2 = 30.7

21 Total Internal Reflection Total internal reflection is when no light escapes; it doesn t refract out of a medium. This typically occurs when the index of refraction is much higher in the first material and/or if the incident angle is large.

22 Total Internal Reflection Fiber optic cables use total internal reflection to transmit information using light. It is a much more efficient way of communicating. Google has set up networks in some cities (Google Fiber). Their intemet speed is up around 1 Gb/s.