A beam of light from a source $L$ is incident normally on a plane mirror fixed at a certain distance $x$ from the source. The beam is reflected back as a spot on a scale placed just above the source $L$. When the mirror is rotated through a small angle $\theta$, the spot of the light is found to move through a distance $y$ on the scale. The angle $\theta$ is given by

• Correct answer
  A

  $\frac{y}{2x}$
• B

  $\frac{y}{x}$
• C

  $\frac{x}{2y}$
• D

  $\frac{x}{y}$

Solution

When mirror is rotated by $\theta$ angle reflected ray will be rotated by $2\theta$ .



for small $\text{θ,}$ $\frac{y}{x}=2\theta$

$\theta =\frac{y}{2x}$

Hence, the correct answer is option: 'A'.