Question

A convex lens is used to form an object on a screen, If the upper half of the lens is blackened so that it becomes opaque, then
A) Only half of the image will be visible
B) The image position shifts towards the lens
C) The image position shifts away from the lens
D) The brightness of the image reduces

Answer 1

Intensity is the amount of energy per unit area taken per unit time. Since it is given that the upper half of the lens is blackened, it is similar to the case if the lens is covered with a black sheet or paper. Therefore, the concept used will be, what happens to the image, and how its nature varies if the only half the lens is visible.

Complete step by step answer:
When the upper half of the lens is blackened, then all the light rays that rays striking at the surface of the lens, will not be reflected. This is because only half of the light rays will be able to pass through the lens from the lower part of the lens.

This will reduce the intensity of the image formed. The image formed will have less brightness.’

If the upper half of the image is blackened, the brightness of the image will decrease.
Therefore, option (D) is the right answer.

Note:
It is to be noted that the brightness or intensity of the image formed in a convex lens also depends on the position of the object. If the object is placed very close to the lens, then also the image formed will be of less intensity. Therefore it can be said that the intensity of light passing through the lens depends on the aperture. Aperture is the opening of the hole through which light rays travel. If the aperture is increased, the intensity of light increases and hence the brightness.