Which of the following are uses of Huygens principle?
1. Finding the shape of the wavefront at a later time.
2. Deriving the laws of reflection and refraction.
3. Finding the energy of a photon of light.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1 and 2
By Huygens principle, each point of a wavefront is the source of secondary wavelets. The forward envelope (common tangent) of these wavelets gives the new wavefront.
1. Finding the shape of the wavefront at a later time. — True. Given the wavefront at \( t=0 \), the envelope of the secondary wavelets after a time \( \tau \) is the wavefront at that later time. The principle is a geometrical construction made for this purpose.
2. Deriving the laws of reflection and refraction. — True. The reflected and refracted wavefronts are drawn with secondary wavelets, which travel with the speed of light in each medium. This gives the laws of reflection and Snell’s law of refraction.
3. Finding the energy of a photon of light. — False. Huygens construction is a wave picture and deals only with the shapes of wavefronts. The energy of a photon, \( E=h\nu \), comes from the photon picture of light.
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.”
Introduction – “In this chapter we will first discuss the original formulation of the Huygens principle and derive the laws of reflection and refraction.”