How did Hippolyte Fizeau determine the speed of light?

Using Huygen's principle, show that the angle of incidence is equal to the angle of reflection when a plane wavefront is reflected by a plane surface.

A monochromatic light wave is incident normally on a glass slab of thickness $d$, as shown in the figure. The refractive index of the slab increases linearly from $n_1$ to $n_2$ over the height $h$. Which of the following statement(s) is(are) true about the light wave emerging out of the slab?

Deduce  Snell's law of refraction for a plane wave using Huygens' principle.