# Critical angle

1. May 21, 2005

### Cheman

Critical angle....

Is the formula:

sin(critical angle) = 1/ mu, only true light is moving from one medium into air/ a vacuum?

2. May 21, 2005

### Staff: Mentor

Correct. (Assuming "mu" is the index of refraction.) To generalize it to any two media (as long as $n_2 < n_1$) go to Snell's law: $n_1 \sin \theta_1 = n_2 \sin \theta_2$. The critical angle arises when $\sin \theta_2 = 1$, thus:
$$\sin \theta_{cr} = \frac {n_2}{n_1}$$

When the second medium is air or vacuum, $n_2 = 1$.

Last edited: May 21, 2005