CK-12-Physics - Intermediate

http://www.ck12.org Chapter 20. Geometric Optics

FIGURE 20.15

http://demonstrations.wolfram.com/LightRayPassingThroughATransparentPlate/

Check Your Understanding

1. Light passes from air into crown glass with an angle of incidence 30◦. What is the angle of refraction?

Answer:According toTable20.1, the index of refraction for crown glass is 1.52. Using Snell’s Law, we have

n 1 sinθ 1 =n 2 sinθ 2 → 1 .00 sin 30. 0 ◦= 1 .52 sinθ 2 →

sinθ 2 =

sin 30. 0 ◦

1. 52

= 0. 3289 →θ 2 =sin−^10. 3289 → 19. 2 ◦

θ 2 = 19. 2 ◦

2. Light passes from air into an unknown fluid with an angle of incidence 30◦and an angle of refraction of 21. 6 ◦.
   What is the velocity of light in the unknown medium?

Answer: Using Snell’s Law and the fact that the speed of light in air is 3. 00 × 108 ms, we have:

v 2 sinθ 1 =v 1 sinθ 2 →v 2 sin 30◦=csin 21. 6 ◦→

v 2 =c

sin 21. 6 ◦

sin 30◦

= 0. 736 c→v 2 = 0. 736

(

3. 00 × 108

m

s

)

= 2. 209 × 108 →

v 2 = 2. 21 × 108

m

s

A Special case of Snell’s Law

If the angle of incidence in Snell’s law is 0◦, we have

n 1 sinθ 1 =n 2 sinθ 2 →n 1 ( 0 ) =n 2 sinθ 2 →sinθ 2 = 0 →θ 2 = 0

We conclude that light incident perpendicular to the interface of two media will pass through the second medium
without refracting.