Light waves travel in vacuum along the y-axis. Which of the following may represent the wavefront?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( y=\mathrm{constant} \)
A wavefront is a surface of constant phase. The energy of the wave travels perpendicular to the wavefront. For light travelling along the y-axis, the wavefront must be a plane perpendicular to the y-axis.
Option (1) \( x=\mathrm{constant} \) — Incorrect. This plane is perpendicular to the x-axis, so it is the wavefront of a wave travelling along the x-axis.
Option (2) \( y=\mathrm{constant} \) — Correct. This plane is perpendicular to the y-axis. All its points are at the same distance along the direction of travel. So they vibrate in the same phase, and the wave moves along the normal to it.
Option (3) \( z=\mathrm{constant} \) — Incorrect. This plane is perpendicular to the z-axis, so it is the wavefront of a wave travelling along the z-axis.
Option (4) \( x+y+z=\mathrm{constant} \) — Incorrect. This plane is perpendicular to the direction \( \hat{\imath}+\hat{\jmath}+\hat{k} \), which is equally inclined to the three axes. So it belongs to a wave travelling along that direction. It is not the wavefront of a wave along the y-axis.
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 speed with which the wavefront moves outwards from the source is called the speed of the wave. The energy of the wave travels in a direction perpendicular to the wavefront.”