# Phase difference and number of wavelengths

1. Oct 9, 2007

### inner08

1. The problem statement, all variables and given/known data
A light wave is propagated from point A to point B in space. We introduce along the way a glass lens with parallel faces of index 1.5 and width L=1mm. The value of teh wavelength is 500μm in space. How many wavelengths are between A and B with and without the glass lens? What is the phase difference introduced as we intercalate the glass lens?

2. Relevant equations
f=v/lambda
v=c/n

3. The attempt at a solution

With the lens, I just used f=v/lambda which gave me f = 6x10^14 Hz.
Without the lens, I first used v = c/n and then used v in f=v/lambda to get 4x10^14 Hz.

I hope I atleast got the first part right. I'm not sure how to go about finding the difference in phase. Any pointers?

2. Oct 9, 2007

### Staff: Mentor

What's the distance between A and B? You are given the wavelength in vacuum (although I suspect that should be nm, not μm) so just divide into the distance to find the number of wavelengths.

How does the wavelength change when the light passes through the glass?

3. Oct 9, 2007

### inner08

I don't have the distance between both points. Is there a way to calculate it or just use a variable?

4. Oct 9, 2007

### Staff: Mentor

I see no way to calculate it. Are you sure you are presenting the problem exactly as given?

But you can certainly calculate the phase difference.

5. Oct 9, 2007

### inner08

yep. I mean, I did translate it from French but I'm usually pretty good at it. It doesn't mention any sort of distance or anything. Just says "from point A to B".

In regards to the phase difference:

with the lense I found: 500nm
without: 750nm

so the phase difference is 750/500 = 1.5?

6. Oct 9, 2007

### Staff: Mentor

I don't quite understand what you've calculated. In vacuum, how many wavelengths are in the 1 mm width? And in glass?