Question

State Huygens' principle of secondary wavelets. With the help of this principle, explain the laws of reflection of light.

Hint:

Huygens' principle is a powerful tool for understanding wave behavior, not only for reflection but also for refraction and diffraction. Always remember that the new wavefront is formed by the envelope of secondary wavelets emitted by each point of the incident wavefront.

Answer 1

Huygens' Principle states that every point on a wavefront can be considered as a source of secondary wavelets, and the position of the wavefront at any later time is the surface tangent to these secondary wavelets. In other words, each point on the wavefront acts as a secondary source of waves, and the new wavefront is formed by the envelope of all these secondary waves.
This principle can be used to explain the laws of reflection of light:
1. Law of Reflection (First Law):
The angle of incidence (\(\theta_i\)) is equal to the angle of reflection (\(\theta_r\)), i.e., \[ \theta_i = \theta_r \] This is confirmed by Huygens' principle because, when the wavefront strikes a reflective surface, each point on the wavefront produces secondary wavelets. These wavelets propagate and form a new wavefront that follows the law of reflection.
2. Law of Reflection (Second Law):
The incident ray, the reflected ray, and the normal to the surface of reflection all lie in the same plane. This can be understood by imagining the wavelets from the incident wavefront interacting with the surface, forming a new wavefront that obeys this geometrical constraint.
Thus, Huygens' principle provides a clear explanation for the reflection of light, where the wavefront interacts with the reflecting surface, and the reflected wavefront is formed by the secondary wavelets.