Question

A flat mirror $M$ is arranged parallel to a wall $W$ at a distance $L$ from it as shown in the figure. The light produced by a point source $S$ kept on the wall is reflected by the mirror and produces a light patch on the wall. If the mirror moves with velocity $v$ towards the wall.

• A. The patch of light will not move on the wall.
• B. The width of the light patch on the wall remains the same.
• C. As the mirror comes closer the patch of light will become larger and shift away from the wall with speed larger than $v$.
• D. The patch of light will move with the speed $v$ on the wall.

Answer 1

Answer: (A), (B)

The width of the light patch on the wall remains the same.


From figure, $SP=PA$ and $SQ=QB$

so, position of $A$ and $B$ doesn't depend on separation of mirror from the wall.

so, the patch $AB$ will not move on the wall.

As, $SA$ and $SB$ are constant we can say that , $AB=$ constant.

Hence, options (b) and (d) are correct.