# Snell's Law for Light and Sound

1. Jan 11, 2012

### sparkle123

A laser beam and a sound wave from directional sources both enter a liquid at an angle of 60 degrees from the horizontal surface of the liquid. The speed of sound in the liquid is 1.8 times that of the speed of sound in air. For light, the refractive index of the liquid is 1.8. What happens?

Solution: The light refracts to an angle of 74° from the horizontal and the sound refracts to an angle of 26° from the horizontal.

I get that the beam and sound wave refract by the same amount since that both have a refractive index of 1.8 for the liquid. Also, my angle is different --> I get 69 degrees from the horizontal.
basically sin (40) = 1.8 sin (θ)
θ=21

2. Jan 11, 2012

### technician

If you are going to use the conventional equations in refraction then the angles should from the NORMAL.... have you done that?
You should also read the information very carefully.
In one case they have given you speed information and in the other case they have given you a 'refractive index'.... do you know the link between speed and refractive index?

3. Jan 11, 2012

### sparkle123

Yes, I did use the normal angles. I though speed is proportional to refractive index, like
v1/v2=n1/n2

4. Jan 11, 2012

### I like Serena

Hey sparkle123!

Speed is inversely proportional to refractive index.

And if the angle with the horizontal is 60 degrees, I'm afraid the angle with the normal is not 40 degrees...

5. Jan 11, 2012

### sparkle123

Wow I'm being awfully stupid! Thanks so much technician and I like Serena! :D