Problem 12

  Refer to Conceptual Example 7 as an aid in understanding this problem. The drawing shows a ray of light traveling through a gas (n = 1.00), a solid (n = 1.55), and a liquid (n = 1.55). At what angle $\theta$ does the light enter the liquid?

SOLUTION: In general, you must apply Snell's Law at each interface between two media of different index of refraction. And be careful what angle you use. Always use the angle away from the normal to the plane separating the media. That being said, we apply Snell's Law at the gas-solid interface:

$$1.00 \sin35.0^o = 1.55 \sin\theta_2$$

$$\theta_2 = 21.7^o$$

Now, we apply Snell's Law again to the solid-liquid interface. But wait, the index of refraction of the solid is the same as that of the liquid. Hence, using Snell's Law, we find that the angle of incidence must equal the angle of refraction. Therefore, the light enters the liquid at an angle of 21.7^o