Subject: why do different wavelengths refract differently?

in my class we have learned that light waves will refract when 
they enter a dispersive medium, and that the higher the wave 
length the more they will bend twards the normal. i was 
wondering what causes light waves with higher frequencies and 
smaller wavelengths to refract more when entering such a medium. 
My teacher was unsure so i was wondering if you could maybe help.

Re: why do different wavelengths refract differently?