Question

A point object at a distance of 36 cm from the convex lens of focal length 10 cm, is moved by 10 cm in 2 sec along principle axis towards the lens. Then image will also change its position.

1. Write the law which relates object and image distance from the lens.

2. Find the initial and final position of the image and calculate average speed of image.

3. A man argues that the image will move uniformly at the same speed as that of object. What is your opinion? Justify.

Answer 1

1. The lens equation \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\)

2. u = -36, f = 10

\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)

i.e. v = \(\frac{36\times10}{46}\) = 7.8 cm

If object is moved 10 cm towards the lens we can find position

u = -26, f = 10

v = 7.2 cm

Speed = \(\frac{7.8-7.2}{2}\) = 3 cm/sec

3. Comparing speed of object and image we can arrive at conclusion that the argument of man is false, speed of image is different from speed of object.