A beam of white light is incident on a thick glass plate with parallel sides, at an angle between e and 90 e with the normal. Which color emerges from the other side first?

Huygen's principle: Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets - that is, tangent to all of them.