Define the term wave front. Using Huygens’s wave theory, verify the law of reflection.
OR
Define the term, “refractive index” of a medium. Verify Snell’s law of refraction when a plane wave front is propagating from a denser to a rarer medium.

A wave front is the continuous locus of vibrating particles which are in the same state of
vibration or phase.
Laws of Reflection from Huygens principle

Consider a plane wave front AB incident on a plane reflecting surface PQ.
Let v be the velocity of the wave.
At time t – 0 one end of the wave front just touches the reflecting surface at B.
Draw normal NB to PQ.
When the wave front strikes the reflecting surface, then due to the presence of it, it cannot
advance further.
When wave front strikes at B, secondary wavelets starts emitting from B.
The secondary wavelets will travel a distance AD = vt during the time the other end A of the
wave front AB reaches the surface PQ at Q.
To find the reflected wave front, B as a center and AD as radius draw an arc, which represent the secondary wavelets originating from B.
As the incident wave front AB advances, the secondary wavelets will touch CD simultaneously.
According to Huygens’s principle CD represents the reflected wave front corresponding to incident wave front AB. BD is common triangles BAD and CBD and BC = AD = vt.
Therefore, two triangles are congruent.
So, $\mathrm{\angle }\mathrm{ABD}=\mathrm{\angle }\mathrm{CDB}$
$\text{ i. e., }\mathrm{\angle }\mathrm{i}=\mathrm{\angle }\mathrm{r}$
Thus, angle of incidence is equal to the angle of reflection. This is the second law of reflection.
Also, the incident wave front AB, reflecting surface PQ and the reflected wave front CD are perpendicular to the plane of the paper.
So, the incident ray, reflected ray and the normal at the point of incidence, all lie in same plane. This is first law of reflection.

OR

Thus, Huygens’s principle explains both the law of reflection.
The refractive index of a medium is defined as the ratio of speed of light in vacuum to that of the speed of the light in medium.
i.e n= c/v
Where, n = refractive index of medium when light ray passes from vaccum into a medium.
c = velocity of light in vacuum
v = velocity of light in the medium
Proof of Snell’s law of refraction