Huygens’ concept of secondary wavelets

The correct answer is Option (2) → is a geometrical method to find a wavefront

Option (1) allows us to find the focal length of a thick lens — Incorrect. The focal length of a lens comes from the lens maker’s formula, worked out for thin lenses. Huygens’ construction only traces how a wavefront moves forward.

Option (2) is a geometrical method to find a wavefront — Correct. Every point of a wavefront acts as a source of secondary wavelets, which spread out with the speed of the wave. The common tangent to these wavelets gives the new wavefront at a later time. Given one wavefront, this construction finds the next.

Option (3) is used to determine the speed of light — Incorrect. The wavelets are drawn with the speed of the wave already known (radius \( v\tau \)). The construction uses the speed and does not measure it.

Option (4) is used to explain the photoelectric effect — Incorrect. The photoelectric effect needs the photon picture of light. A wave picture cannot explain the threshold frequency or the instant emission.

NCERT (2026-27):
Huygens Principle – “… Now, if we know the shape of the wavefront at t = 0, then Huygens principle allows us to determine the shape of the wavefront at a later time τ. Thus, Huygens principle is essentially a geometrical construction, which given the shape of the wafefront at any time allows us to determine the shape of the wavefront at a later time.”