Cracking the SAT Physics Subject Test

2) θ 1 > θc, where θc = sin−1 (n 2 /n 1 )

Keep Your Instances

and Incidences Straight

Think of n 1 or θ 1 as the

incidence ray, or where

the source comes from.

Treat n 2 or θ 2 as the

resulting ray.

Notice that total internal reflection cannot occur if n 1 < n 2. If n 1 > n 2 , then total

internal reflection is a possibility; it will occur if the angle of incidence is large
enough, that is, if it’s greater than the critical angle, θc.

Questions 7-8

The critical angle for total internal reflection between air and water

is known to be 49°.

7. If a beam of light striking an air/water boundary undergoes total
   internal reflection, will it stay in the air or in the water?


8. Describe what happens if a beam of light in the air strikes the
   surface of a calm body of water at an angle of 50° to the normal.

A Trick of the Light

For an optics problem,

remember to check

whether total internal

reflection occurs. If it does,

then there’s no refracted

ray. Trick problems may

ask for the angle of

refraction in a situation

like this, but in this case,

there simply isn’t an angle

of refraction.

Here’s How to Crack It

7. Total internal reflection can occur only if the light is in the medium with