A point source of light S is placed at a distance 10 cm in front of the centre of a mirror of width 20 cm suspended vertically on a wall. An insect walks with a speed `10cm//s` in front of the mirror along a line parallel to the mirror at a distance 20 cm from it as shown in figure. find the maximum time during which the insect can see the image of the source S in the mirror.

Answer

6 s

Explanation

Insect can see the image of source S in the mirror, so far as it remains iin fields of view of image overlapping with the road.
Shaded portion is the field of view. Which overlaps with the road upto lenght PQ.
By geometry we can see that, `PQ=3AB=60cm`
`:. t=("distance")/("Speed")=60/10 =6s`.