Question

If half part of a convex lens is covered, the focal length____ charge, but the intensity of the image___.
(A) Does, increase
(B) Does not, increase
(C) Does, decreases
(D) Does not, decreases

Answer 1

Hint: In this question, the concept of the brightness of the image of the convex lens will be used, that is brightness of the image decreases when some part of the lens is blocked so we can see that the lesser number of rays passes through the lens.

Complete step by step answer:
In this question, we have given half part of the convex lens and we have to consider the focal length and the intensity of the image.
We know that the focal length is shown as the distance from poles and center of curvature. The intensity of the image is shown the size of the image.
From the experiment we have observations that, smaller lens can also form a complete image of an object placed in front of it. However, brightness of the image decreases when some part of the lens is blocked so we can see that the lesser number of rays passes through the lens.

Now we consider that the half part of a convex lens is covered as shown in the figure (1)


Figure (1)
Therefore, the half part of a convex lens is covered; the focal length does not change, but the intensity of the image decreases.
Hence, the black of the given statement is filled with does not and decreases.
Thus, the correct option is (D).

Note: We have recognized the convex lens focal length does not change, if we change focal length of the lens then that image intensity will increase. So a convex lens always has the same focal length that shows whether the image of the object is converted or not.