Pink Floyd's famous album the Dark Side of the Moon has on its cover an illustration of…
If light is a wave and glass is something that changes the wavelength of light as it moves through it, then Snell’s Law should apply.
Blue light, associated with short wave lengths, should refract more than red light. Blue light should therefore end up at a different relative location from red light. The result of this would be a rainbow effect in which colors get blurred and mixed.
Viewed at an angle, a stick painted red and blue would have the appearance of either splitting apart or contracting together into a shorter stick. Multi colored fishes in aquariums would appear blurred and miss-colored.
Applying Snell’s Law to light going through a glass sheet
None of this happens, not even for very thick sheets of glass. Yet the tiniest speck of broken glass can produce a fantastic little rainbow. Diffraction of light is purely a function of relative surfaces. If the surface penetrated on entering the glass is parallel to the exit surface, no diffraction takes place.
Diffraction only happens when the surface of entry and the surface of exit are at an angel to each other. Diffraction only happens on exit from a medium. This is contrary to Snell’s Law, and proof that light cannot be treated purely as waves.