question_answer

The mirror of length L moves horizontally as shown in the figure with a velocity v. The mirror is illuminated by a point source of light 'P' placed on the ground. The rate at which the length of the light spot on the ground increases is -

A) v

B) zero 

C) 2v

D) 3v

Correct Answer: B

Solution :

\[{{V}_{A}}=\frac{d}{dt}(2vt)=2v;{{v}_{B}}=\frac{d}{dt}(2vt+2L)=2v\]

\[\Rightarrow {{v}_{A}}={{v}_{B}}\]

\[\therefore \]Length of AB will remain same.