# Physics: intensity/double slits help

1. Nov 25, 2004

### vtnsx

I'm so lost... can someone please explain :( thanx

Red light with wavelength 700 nm is passed through a two-slit apparatus. At teh same time, monochromatic visible light with another wvelength passes through teh same apparatus. As a result, most of the pattern that appears on the screen is a mixture of two colors; however, the center of the third bright fringe (m=3) of the red light appears pure red, with none of other color. What are the possible wavelengths of the second type of visible light? Do you need to know the slit spacing to answer this question? Why or why not?

2. Nov 25, 2004

### learningphysics

Hmmm... I think I understand the question. Your physics text should have a derivation for the distance of a bright fringe from the central bright fringe.

1) Find the distance (or a formula for the distance) for the 3rd bright fringe of the red light.

2) The other wavelength of light cannot have a fringe at the same distance as what the distance you found in part 1 (since the center of the 3rd fringe is all red). Use this fact to obtain the possible values of the other wavelength. You'll be using the same distance equation. (hint: what wavelengths can the other light NOT have)

From 1) and 2) you should be able to find out whether or not the slit spacing matters.