Identify the correct statements among the following.
(A) The wavefront for a point source of light is in a spherical shape, rays are all radial from the point source.
(B) All the points on a cylindrical wavefront vibrate in the same phase.
(C) A set of straight lines perpendicular to a wavefront are called rays of light. Light energy does not flow along these lines.
(D) Every point of a given wavefront acts as a fresh source of a secondary wavelet.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B) and (D) only
(A) The wavefront for a point source of light is in a spherical shape, rays are all radial from the point source. — Correct. A point source sends light equally in all directions, so the surface of constant phase is a sphere and the rays, being normal to it, are radial.
(B) All the points on a cylindrical wavefront vibrate in the same phase. — Correct. A wavefront is by definition a surface of constant phase, whatever its shape.
(C) A set of straight lines perpendicular to a wavefront are called rays of light. Light energy does not flow along these lines. — Incorrect. The first sentence is right, but the energy of light does flow along the rays, since they show the direction of propagation.
(D) Every point of a given wavefront acts as a fresh source of a secondary wavelet. — Correct. This is Huygens' principle.
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. … The energy of the wave travels in a direction perpendicular to the wavefront.”
Huygens Principle – “… If we have a point source emitting waves uniformly in all directions, then the locus of points which have the same amplitude and vibrate in the same phase are spheres and we have what is known as a spherical wave as shown in Fig. 10.1(a).”
Summary – “1. Huygens’ principle tells us that each point on a wavefront is a source of secondary waves, which add up to give the wavefront at a later time.”