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 :

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

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

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