Refraction

1. Refraction Physics 1161: Lecture 22 • Textbook sections 26-3 – 26-5, 26-8

2. Wave Boundary Behavior • wave speed and wavelength are greater in less dense medium • wave frequency is not altered by crossing boundary • reflected pulse is inverted when wave travels from less dense medium to more dense medium • incident pulse amplitude is greater than reflected pulse amplitude

3. Transmission Across a Boundary • Wave speed & wavelength change • When the wave approach is perpendicular to the boundary, its speed changes, but there is no bending of the path

4. Refraction of Light Beam • Refraction -- bending of light wave path as light passes from one material to another material. • Refraction occurs at the boundary and is caused by a change in the speed of the light wave upon crossing the boundary. • Direction of bending depends upon whether light wave speeds up or slows down at the boundary.

5. Optical Density • Optical density -- tendency of the atoms of a material to hold on to absorbed energy from a photon in the form of vibrating electrons before reemitting it as a new photon • The more optically dense a material is, the slower a wave will move through the material.

6. Index of Refraction • Index of Refraction is a measure of optical density • Represented by n • The higher n is, the more optically dense the material and the slower light travels in the material

7. Indices of Refraction Physics 1161: Lecture 17, Slide 7

8. Law of RefractionSnell’s Law n1sinq1 = n2sinq2

9. Refraction Applets • Applet by Molecular Expressions -- Florida State University • Applet by Fu-Kwung Hwang, National Taiwan Normal University

10. FST & SFA • A ray of light crossing the boundary from a fast medium to a slow medium bends toward the normal. (FST) • A ray of light crossing the boundary from a slow medium to a fast medium bends away from the normal. (SFA)

11. Snell’s Law When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n1 sin(q1)= n2 sin(q2) Preflight 22.1 n1 q1 1) n1 > n2 2) n1 = n2 3) n1 < n2 21 % q2 n2 21 % Compare n1 to n2. 58 %

12. Snell’s Law When light travels from one medium the speed changes. If the angle of incidence is greater than 0, the light bends. During this process, the frequency remains constant. n1 sin(q1)= n2 sin(q2) Preflight 22.1 n1 q1 1) n1 > n2 2) n1 = n2 3) n1 < n2 q1 < q2 q2 n2 sinq1< sinq2 Compare n1 to n2. n1> n2

13. Example Snell’s Law Practice 1 r Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60. The other part of the beam is refracted. What is q2? n1 n2 normal

14. Example Snell’s Law Practice 1 r Usually, there is both reflection and refraction! A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60. The other part of the beam is refracted. What is q2? q1 =qr =60 sin(60) = 1.33 sin(q2) n1 q2 = 40.6 degrees n2 normal

15. air air 1 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? • Medium 1 • Medium 2 • Both the same 2

16. air air 1 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? • Medium 1 • Medium 2 • Both the same The greater the difference in the speed of light between the two media, the greater the bending of the light rays. 2

17. 1 2 3 Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the indices of refraction of these media? 1. n1 > n2 > n3 2. n3 > n2 > n1 3. n2 > n3 > n1 4. n1 > n3 > n2 5. none of the above

18. 1 2 3 Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the indices of refraction of these media? 1. n1 > n2 > n3 2. n3 > n2 > n1 3. n2 > n3 > n1 4. n1 > n3 > n2 5. none of the above Rays arebent toward the normalwhen crossing into #2, son2 > n1. But rays arebent away from the normalwhen going into #3, son3 < n2. How to find the relationship between #1 and #3? Ignore medium #2! So the rays arebent away from the normalif they would pass from #1 directly into #3. Thus, we have:n2 > n1 > n3 .

19. Apparent Depth • Light exits into medium (air) of lower index of refraction,  and turns left.

20. Spear-Fishing • Spear-fishing is made more difficult by the bending of light. • To spear the fish in the figure, one must aim at a spot in front of the apparent location of the fish.

21. To spear a fish, should you aim directly at the image, slightly above, or slightly below? 1. aim directly at the image 2. aim slightly above 3. aim slightly below

22. To spear a fish, should you aim directly at the image, slightly above, or slightly below? 1. aim directly at the image 2. aim slightly above 3. aim slightly below Due to refraction, the image will appearhigherthan the actual fish, so you have toaimlowerto compensate.

23. To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below? 1. aim directly at the image 2. aim slightly above 3. aim slightly below light from fish laser beam The lightfrom the laser beam will alsobendwhen it hits the air-water interface, soaimdirectly at the fish.

24. Delayed Sunset • The sun actually falls below below the horizon • It "sets", a few seconds before we see it set.

25. Broken Pencil

26. Water on the Road Mirage

27. Palm Tree Mirage

28. Mirage Near Dana – Home of Ernie Pyle