15.1 Refraction

1. 15.1 Refraction pp. 562 -567 Mr. Richter

2. Agenda • Warm-Up • Introduction to Refraction Demo • Reminder: Quarter 3 Ends Friday • Notes: • Refraction • Index of Refraction • Why Light Bends • Refracted Images • Snell’s Law

3. Objectives: We Will Be Able To… • Recognize situations in which refraction will occur. • Identify which direction light will bend when it passes from one medium to another. • Solve problems using Snell’s law.

4. Warm-Up: • When a pencil is partially submerged, it looks like it bends at the surface of the water. • What is really happening?

5. Refraction

6. Refraction • When light encounters a boundary, some of the light is reflected and, depending on the boundary, some of the light is refracted. • Refraction is the bending of a ray of light as it passes from one boundary to another. • When light is absorbed, it is not refracted.

7. Refraction • The angle of refraction θr(how much the light bends) depends on: • The media (materials) through which the light travels • The angle at which light strikes the boundary

8. Refraction • Your book says: • “Refraction occurs when light’s velocity changes” • WRONG! OMG! • Light always travels at the speed of light, no matter what. • However, it gets absorbed and reemitted more in some mediums than others, so its “net speed” can be lessened.

9. Index of Refraction

10. The Index of Refraction • Every material bends light a different amount. • The material’s index of refraction of a material indicates how much light will be bent passing through that material. • The index of refraction of a medium is the ratio of the speed of light in a vacuum to the “speed” of light through the medium.

11. The Index of Refraction • Note the indices of refraction to the right (p. 564). • Air bends light very little, while diamonds bend light very well. • Higher index = more bending! • No indices below 1.

12. Wavelength and Refraction • Table 15-1 is only valid for wavelengths of 589 nm in a vacuum. • Different wavelengths have different indices of refraction. • This is why white light separates in a prism. Different colors have different wavelengths.

13. Why Light Bends

14. Index of Refraction: Why Light Bends • Imagine your right arm as a wave. • As you walk, your arm moves with you through the air. • However, if you drag your hand along a fence, your arm drags behind, bending your body.

15. Index of Refraction: Why Light Bends • The same thing happens with waves of light. • As wave fronts encounter a new material, they will slow down (or speed up), bending the wave.

16. Refracted Images

17. Refracted Images • Objects appear to be at different positions due to refraction. • If light coming from the object (incident) goes to material with a lower index of refraction, the object appears farther from the normal line. • If ni > nr, then θi < θr

18. Refracted Images • The waves of light leaving the fish travels at one angle. • But as the light hits the boundary (air to water), part of the wave can move faster. • This changes the angle of the light that the cat sees, making the fish appear farther from the normal line. • And higher up.

19. Refracted Images • If light coming from the object (incident) goes to material with a higher index of refraction, the object appears closer to the normal line. • If ni < nr, then θi > θr

20. Snell’s Law

21. Snell’s Law • Snell’s law determines the angle of refraction based on: • the angle of incident light • the indices of refraction from the two different media

22. Practice Problem • A light ray traveling through air strikes a smooth slab of crown glass at an angle of 30.0° to the normal. Find the angle of refraction, θr. • Given: • θi = 30.0° • ni = 1.00 • nr = 1.52 (Table 15-1) • Calculations: θr= 19.2°

23. Wrap-Up: Did we meet our objectives? • Recognize situations in which refraction will occur. • Identify which direction light will bend when it passes from one medium to another. • Solve problems using Snell’s law.

24. Homework • p. 567 #1-4