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.  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 θ r (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.  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 n i > n r, then θ i < θ r

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 n i θ r

20. Snell’s Law

21.  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, θ r.  Given:  θ i = 30.0°  n i = 1.00  n r = 1.52 (Table 15-1)  Calculations: θ r = 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