Physics (optics) question

[dmw16]

A narrow beam of white light is incident at 30.0° onto a slab of heavy flint glass 4.00 cm thick. The indices of refraction of the glass at wavelengths 400 nm and 700 nm are 1.689 and 1.642, respectively. Find the width of the visible beam as it emerges from the slab.

This was a homework problem I had and I could not figure out how to solve it. Can anyone help me?
thanks,
-doug

EDIT: I had been using snell's law and keep getting the wrong answer. Also, I should have mentioned the answer needs to be in mm. I had figured it in degrees, but that doesnt do me much good. What I did try was figuring out the distance apart as they leave the glass. I guess I can take a few more cracks at it in the morning.

 

[zzzz]

lets say its a point source. Also assume the limits of visible light to be 400 and 700nm. Now one wavelength is going to "bend" more than other. Calculate the bending ( Snells Law) for each wavelength at first interface. And apply geometry.

 

[jahawkin]

I think you just use snell's law to figure out the angles that the different wavelength refract at. They get refracted entering and exiting the glass so just draw a diagram and you should get a width expressed in degrees...

 

[AvesPKS]

Heh...the only thing I remember that might be even remotely helpful is Snell's law...