Identify the correct statements among the following in the context of Huygens principle related to wave theory.
A. Wavefront is locus of all points in which light waves are in same phase. Propagation of wave energy is perpendicular to the wave front.
B. A common tangential surface on secondary wavelets, gives new wavefront.
C. Every point of a wavefront becomes secondary source of light.
D. The secondary sources produce secondary wavelets.
E. The secondary wavelets have different speeds and wavelengths.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, B, C and D only
Huygens principle: every point of a wavefront acts as a source of secondary wavelets that spread out with the speed of the wave; the forward envelope (common tangent) of these wavelets gives the new wavefront.
(A) Wavefront is locus of all points in which light waves are in same phase. Propagation of wave energy is perpendicular to the wave front. — Correct. This is the definition of a wavefront; the rays, which show the flow of energy, are normal to it.
(B) A common tangential surface on secondary wavelets, gives new wavefront. — Correct. The forward envelope of the wavelets after time t is the new wavefront.
(C) Every point of a wavefront becomes secondary source of light. — Correct. This is the first postulate of Huygens principle.
(D) The secondary sources produce secondary wavelets. — Correct. Each secondary source sends out spherical wavelets.
(E) The secondary wavelets have different speeds and wavelengths. — Incorrect. In a given medium the wavelets travel with the same speed as the wave and have the same wavelength; that is why the envelope is a proper wavefront.
NCERT (2026-27):
Huygens Principle – “… Such a locus of points, which oscillate in phase is called a wavefront; thus a wavefront is defined as a surface of constant phase.”
Huygens Principle – “… The energy of the wave travels in a direction perpendicular to the wavefront.”
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.”