Question

State Huygens principle. How did Huygens justify the absence of the backwave on a spherical wavefront ?

Hint:

Remember: Huygens proposed secondary wavelets, while Fresnel explained the absence of backwave through interference of those wavelets.

Answer 1

Concept: The wave theory of light proposed by the Dutch physicist Huygens provides a geometrical method for understanding the propagation of wavefronts. Using this principle, one can explain reflection, refraction and propagation of light waves.

Step 1: State Huygens' Principle. Huygens' Principle consists of the following two statements:
• Every point on a given wavefront acts as a source of secondary spherical wavelets.
• The new wavefront at any later instant is the forward envelope (common tangent surface) of all these secondary wavelets. Thus, a wavefront continuously advances through the formation of secondary wavelets.

Step 2: Explain the concept of secondary wavelets. Consider a spherical wavefront emitted from a point source. Each point on this wavefront behaves as a new source and emits secondary spherical wavelets in all directions. The forward envelope of these wavelets gives the next position of the wavefront.

Step 3: Justification for the absence of backwave. If every secondary wavelet propagated equally in all directions, one would expect a backward travelling wave called a backwave. However, such backwaves are not observed experimentally. To explain this, Fresnel modified Huygens' theory. According to the Huygens-Fresnel principle:
• Secondary wavelets interfere with one another.
• In the backward direction, destructive interference occurs.
• In the forward direction, constructive interference occurs. As a result, the resultant intensity in the backward direction becomes negligible. Therefore, no observable backwave is produced. Final Answer:
• Every point on a wavefront acts as a source of secondary spherical wavelets.
• The forward envelope of these wavelets gives the new wavefront.
• The absence of backwave is explained by destructive interference of secondary wavelets in the backward direction and constructive interference in the forward direction.