Arrange the following steps according to the Huygens principle pertaining to the construction of a wavefront at a later time from an initial spherical wavefront at t = 0.
(A) The envelope of the secondary wavelets thus obtained produces the new wavefront.
(B) We draw a common tangent to all these spheres representing the secondary wavelets.
(C) Each point of the initial wavefront is the source of a secondary wavelet.
(D) These secondary wavelets emanating from these points on the initial wavefront spread out in all directions with the speed of the wave.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (C), (D), (B), (A)
Huygens’ construction starts from the known wavefront, treats its points as sources, lets their wavelets spread for a time t, and then joins the wavelets to get the new wavefront.
(C) Each point of the initial wavefront is the source of a secondary wavelet. — comes first: the points of the known wavefront act as sources of secondary wavelets.
(D) These secondary wavelets emanating from these points on the initial wavefront spread out … with the speed of the wave. — comes next: after a time t each wavelet is a sphere of radius vt.
(B) We draw a common tangent to all these spheres representing the secondary wavelets. — follows: a common tangent joins the spheres.
(A) The envelope of the secondary wavelets thus obtained produces the new wavefront. — comes last: this common tangent, the envelope, is the new wavefront.
So the order is (C), (D), (B), (A).
NCERT (2026-27):
Huygens Principle – “… Now, according to Huygens principle, each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions with the speed of the wave. These wavelets emanating from the wavefront are usually referred to as secondary wavelets and if we draw a common tangent to all these spheres, we obtain the new position of the wavefront at a later time.”