# Is Snell's Law valid even when incident ray is Normal to the surface?

1. May 10, 2012

### aleemudasir

According to the Snell's Law refractive index n21= sin i/sin r, but when we use this equation while having a incident light normal to the surface of lens or any other refracting surface it becomes 0/0. So how can we define Snell's law in this situation?

2. May 10, 2012

### Infinitum

$$n_1sin\theta_1 = n_2sin\theta_2$$

This is the actual Snell's law. When you shift the sin over to the other side, you assume it's not 0.

3. May 10, 2012

### aleemudasir

Which one is the actual n21=sin i /sin r or n1 sin i= n2 sin r?

4. May 10, 2012

### Infinitum

As I said above...

$$n_1sin(i) = n_2sin(r)$$

The other one is not valid when sin(r) is equal to zero.