Which one of the following phenomenon is not explained by Huygen's construction of wavefront?

On a hot summer night, the refractive index of air is the smallest near the ground and increases with a height from the ground. When a light beam is directed horizontally, the Huygens' principle leads us to conclude that as it travels, the light beam,

Which of the following are true?

A. Speed of light in vacuum is dependent on the direction of propagation.

B. Speed of light in a medium is independent of the wavelength of light.

C. The speed of light is independent of the motion of the source.

D. The speed of light in a medium is independent of intensity.

Choose the correct answer from the question given below :

What is the phase difference between any two points lying on the same wavefront?

Calculate the longest wavelength that can be analysed by a rock salt crystal of spacing $\mathrm{d}=2.82 \stackrel{\mathrm{o}}{\mathrm{A}}$ in the first order.

State Huygens' principle.

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