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. The mirror moves with velocity v towards the wall.

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

Answer 1

Answer: (B), (D)

The correct options are
B The patch of light will not move on the wall.
D The width of the light patch on the wall remains the same.

Lets draw the ray diagram,

As we know, the image distance is as same as object distance from the mirror, here wall also act as a screen, mirror movement only affect the image's speed towards the mirror, not along the mirror, therefore using basic geometry.

Length of light patch
$=(x+y)+(y-x)$
$=2y$
and remains same