Question on wavelength using the two slit experiment

[meeklobraca]

Homework Statement

Red light with wavelength of 700 nm is passed through a two slit apparatus. At the same time, monochromatic visible light with another wavelength passed through the 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 the 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?

Homework Equations

The Attempt at a Solution

Well the wavelength of the other color has to be somewhere between 400 and 699 nm i would think. But I have no idea how to be more specific than that. I would think you would need to know the slit spacing to be more specific because to use an equation of the two slit experiement, it requires the slit space distance.

What do you guys and girls think?

 

[mgb_phys]

The only way the red fringe could appear pure red is of there is no contribution from any other colour.
So the white light fringes must be a minima (ie dark) at this point

 

[meeklobraca]

So to answer this question I must...?

 

[mgb_phys]

write down some of the equations for a double slit interference setup

 

[ideasrule]

Do you know the equations for maxima & minima in a double slit experiment? Write one equation representing red light and the other representing the unknown wavelength. See if you can solve it from there.

 

[meeklobraca]

I don't really have time for that. Let's assume I have all the equations and that I know of them.