Question

An optical device is constructed by fixing three identical convex lenses of focal lengths 10 cm each inside a hollow tube at equal spacing of 30 cm each. One end of the device is placed 10 cm away from a point source. How much does the image shift when the device is moved away from the source by another 10 cm?

(a) 0

(b) 5 cm

(c) 15 cm

(d) 45 cm

Answer 1

Answer is : (a) 0

Initially given situation is

When device is moved away from source O, then situation is as shown below.

So, distance between object and image in both cases is 90 cm. Hence, there is no shift in image’s position.