Huygens’ principle helps us to know

The correct answer is Option (4) → the propagation of wavefronts of light wave

Huygens’ principle is a geometrical method of finding the position of a wavefront at a later time. Every point on a wavefront acts as a source of secondary wavelets, which spread out with the speed of the wave. The forward envelope of these wavelets after a time t gives the new wavefront. So the principle tells us how wavefronts propagate, and it is used to derive the laws of reflection and refraction. It says nothing about the amplitude, intensity or polarisation of the wave.

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 \( t \). 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.”