Spacing between slits in a diffraction grating w/o known wavelength

[chanderson]

Homework Statement

I have an issue for a question I have. The problem is that I need to find the distance between the slits of a diffraction grating. Normally this would be pretty simple, but I'm not given the wavelength of the light source we're supposed to use to find it out.

Homework Equations

I know X~m(lambda)L / d and X = Ltan(theta) but this is difficult to figure out without the wavelength. I also know that dsin(theta) = m(lambda) but if I substitute that into the inital equation it eliminates d altogether.

The Attempt at a Solution

I know that if given the wavelength I could easily find d by using the initial equation and several simple measurments. I could also use the more accurate Ltan(theta) and measure the angle from the normal to the first maxima. I could set it up by setting the grating a certain distance from a blank wall and shine the light source through it, but without the wavelength I end up with two unknowns so I'm pretty much stuck.

 

[ehild]

Illuminate the slit with white light. You know the limit wavelengths of the visible range.

ehild