Need and explanation of an Optics principle.

[zumbo1]

There is a laser beam with a diameter D, in air that strikes a flat smooth piece of glass with angle @ and index of refraction of n. I need to derive and equation for the diameter of the beam in the glass wiht respect with n D, and @.
I want to know why the beam's diameter would be effected by the index of refraction. Can you give me a hint of what laws and principles I should use.
Anyway I am going to reread my textbook and class notes again to see if I missed anything that could be applied to this situation.
I want to thank you for your help, in advance.

 

[Galileo]

Draw a picture and and use Snell's law. I`m sure you can imagine in the extreme case of infinite refraction index and glancing incidence that the beam in the medium will clearly have a larger diameter.