# Homework Help: Double slit question

1. Feb 10, 2010

### metalmagik

1. The problem statement, all variables and given/known data

A grating with 349 lines/mm is illuminated with light of wavelength 412 nm What is the angular separation between the two lines formed in order m=2?

2. Relevant equations

$$\vartheta$$ = m$$\lambda$$/d

3. The attempt at a solution

I did not know how to use this equation successfully since I did not have d. I do not know what the 349 lines/mm has to do with anything, so that is obviously what I'm missing. Help!

2. Feb 11, 2010

### Stonebridge

This is not a "double slit" question, it is a diffraction grating question, for which there is an appropriate equation. (Not the one you give)
d is the separation between the lines in the grating and is calculated from the number of lines per mm that is given by a simple inverse relationship.

3. Feb 11, 2010

### Redbelly98

Staff Emeritus
That's okay, in diffraction problems the same equation is used for a diffraction grating and the double slit . (You can click on the terms "diffraction grating" and "double slit", in that previous sentence, for more info.)

There are 349 line spacings in 1 mm. Therefore, the distance between 2 adjacent lines is ___?

4. Feb 11, 2010

### metalmagik

1/349! So that is equal to d? I worked the problem out for $$\vartheta$$ and I'm getting like .28, the multiple choice answers are 26, 40, and 33$$^{o}$$...