What is wave normal and wave front?

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,

State Huygens's principle of wave theory?  Using this principle derive the law of reflection or refraction. Which quantity remains unchangedwhen a light wave suffers reflection of refraction?

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.