# Homework Help: Snells law - help please

1. Apr 29, 2014

### Sprattboy

A light ray approaches a glass block at 30° to the normal. The refractive
indices of air and water are 1 and 1.5 respectively At what angle will the light
be refracted?

This uses Snells law

I did Sin 30 / Sin r = 1 / 1.5
I then multiplied both sides in order to get rid of sin 30.
After that, I got Sin r = 1/1.5(xSin30)
This gave me an answer of -1.48
When I do inverse sine it gives me an error
What did I do wrong???

2. Apr 29, 2014

### SammyS

Staff Emeritus

3. Apr 29, 2014

### Sprattboy

I have

4. Apr 29, 2014

### TheAustrian

Check it again and again, just to be sure, a minus answer should be rather unlikely in elementary examples.

5. Apr 29, 2014

### TheAustrian

You should have got this:

$\frac{1}{sin(r)}=\frac{1}{1.5(sin(30))}$

6. Apr 29, 2014

### dauto

Make sure your calculator is set for degrees instead of radians. You're getting a negative sign because you're using radians. That's what happens when you blindly follow the calculator without a clue of what to expect. You end up eating calculator garbage.

7. Apr 30, 2014