Question

A plane mirror is approaching a person at a speed of $5cm{s}^{-1}$. At what speed will his image approach him?

• A. $10cm{s}^{-1}$
• B. $5cm{s}^{-1}$
• C. $20cm{s}^{-1}$
• D. $15cm{s}^{-1}$

Answer 1

Answer: (A)

$10cm{s}^{-1}$

Let us consider that the object distance from the mirror is $u=a$

As in the case of a plane mirror, the image distance is equal to the object distance is same, hence

$v=a$

Therefore the distance between the object and image will be $d=2a$

As the mirror is approaching at the speed of $s_o=\frac{d_o}{t}=5cm/s$, hence

The speed of the image will be twice $s_i=\frac{d_i}{t}=2\times 5cm/s=10cm/s$.

Hence the image is approaching at a speed of $10cm{s}^{-1}$ .