# Propagation of Light

1. Apr 22, 2005

### sghaussi

Hi! I was wondering if you can give me some adivice on how to approach this problem:

In a physics lab, light with a wavelength of 560 nm travels in air from a laser to a photocell in a time of 16.5 ns. When a slab of glass with a thickness of 0.860 m is placed in the light beam, with the beam incident along the normal to the parallel faces of the slab, it takes the light a time of 21.3 ns to travel from the laser to the photocell.

What is the wavelength of the light in the glass?
Use 3×108 m/s for the speed of light in a vacuum.

My main problem is that I don't know how the thickness of the medium is important.

Thank you in advance,
Sahar

2. Apr 22, 2005

how long did the light take to travel the glass?
thats when the thickness counts
you can get your index once you figure this out
as the light goes through the glass, the frequency doesn't change just the wavelength
what does this tell you?

Last edited: Apr 22, 2005
3. Apr 22, 2005

### Andrew Mason

The first thing to do is to find how long the original path is.

$$s_0 = c\Delta t_0$$

For the path through the glass, there are two parts:

$$s_{air} = c\Delta t_{air}$$ and

$$s_{glass} = v_{glass}\Delta t_{glass}$$

so you know, or can work out: $s_{air},\Delta t_{air}, s_{glass}, \Delta t_{glass}$

From that you should be able to work out $v_{glass}$ and wavelength follows from that.

AM

Last edited: Apr 22, 2005