Question

If the object is placed at 60 cm from the concave mirror. The object moves with velocity 20 $\frac{cm}{sec}$ towards the mirror. The magnification is $\frac{-1}{2}$ at initial position. And if magnification after 1 sec is – 1. Find out the average velocity of the image for above 1 sec period.

• A. 10 cm/sec
• B. 20 cm/sec
• C. 15 cm/sec
• D. 5 cm/sec

Answer 1

The correct option is A

10 cm/sec

m = $\frac{-v}{u}$

$\frac{-1}{2}$ = $\frac{\left(v\right)}{\left(60\right)}$

v = - 30 cm

In 1 sec, object will move 20 cm towards the mirror. So, final object position will be 40 cm before the mirror.

After 1 sec = 1 = $\frac{v}{(-40)}$

v = - 40 cm

So, average velocity = $\frac{(-40)-(-30)}{\left(1\right)}$

= - 10 $\frac{cm}{s}$