What is the shape of the wavefront from a point source kept under water?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Spherical
Option (1) Spherical — Correct. Water is a uniform medium, so light from the point source travels with the same speed in all directions. At any instant the points that vibrate in the same phase are all at the same distance from the source. They lie on a sphere, so the wavefronts are spherical.
Option (2) Cylindrical — Incorrect. The points of a cylinder are at equal distance from a line. They are not at equal distance from a point. So a cylindrical wavefront cannot come from a single point source.
Option (3) Planar — Incorrect. A plane wavefront is only a small portion of a spherical wavefront at a large distance from the source. The wavefront spreading from the source itself is curved.
Option (4) Conical — Incorrect. The points on a cone lie at different distances from its tip. So waves spreading from a point at one speed cannot reach them all in the same phase.
NCERT (2026-27):
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).”
Fig. 10.1: (a) A diverging spherical wave emanating from a point source. The wavefronts are spherical.