Which of the following statements about wavefronts is NOT correct?

The correct answer is Option (2) → Two points on the same wavefront that are half a wavelength apart oscillate in opposite phase.

A wavefront is the locus of points that oscillate in phase. Points half a wavelength apart differ in phase by \( \pi \) only when they lie one behind the other along the direction of travel. Such points are on different wavefronts.

Option (1) A wavefront is a surface of constant phase. — Correct statement. All the points of a wavefront are at the same phase of their oscillation. This is how a wavefront is defined.

Option (2) Two points on the same wavefront that are half a wavelength apart oscillate in opposite phase. — Not correct. Every point on one wavefront oscillates in the same phase, however far apart the points are along it. The phase difference between any two of them is zero.

Option (3) The speed with which a wavefront moves outwards from the source is the speed of the wave. — Correct statement. The wave advances as its wavefronts move outwards, so the speed of the wavefront is the speed of the wave.

Option (4) Far away from a point source, a small portion of the spherical wavefront can be treated as a plane wavefront. — Correct statement. At a large distance the radius of the sphere is very large. So a small part of it is nearly flat. This is why light from a distant source arrives as a plane wave.

NCERT (2026-27):
Huygens Principle – “… Clearly, all points on such a circle are oscillating in phase because they are at the same distance from the source. 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 speed with which the wavefront moves outwards from the source is called the speed of the wave.”

Huygens Principle – “… At a large distance from the source, a small portion of the sphere can be considered as a plane and we have what is known as a plane wave ….”