Question

Light waves are travelling in vaccum, along the $y$-axis. Which of the following, may represent a plane wavefront?

• A. $x=c$
• B. $y=c$
• C. $z=c$
• D. $x+y+z=c$

Answer 1

The correct option is B $y=c$

Wavefronts are formed by joining all the points of the medium vibrating in the same phase, at a given instant. The wavefront and direction of propagation of light are mutually perpendicular. So, for light waves travelling along $y$-axis, the wavefront would be $y=c$.

Hence, $\left(B\right)$ is the correct answer.

$\begin{array}{c}\text{Why this question?}\\ \text{Note:- The wavefronts for a wave going along a fixed direction are planes perpendicular to that direction.}\end{array}$