Consider two diffraction gratings; one has 4000 lines per cm and the other one has 6000 lines per cm. Make a statement comparing the dispersion of the two gratings.

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.