Question

A large lens is used to focus an image of an object onto a screen. If the left half of the lens is covered with a dark card, which of the following occurs

(A) The left half of the image disappears 

(B) The right half of the image disappears 

(C) The image becomes blurred

(D) The image becomes dimmer

(E) No image is formed

Answer 1

The correct option is: (D) The image becomes dimmer

Explanation:

When light from multiple locations pass through a given part of a lens to form an image, only a small portion of a lens is needed to form the image. The more of a lens, the more light rays that can be bent by it to each image location. This simply makes the image brighter. By covering half the lens, all of the incoming rays still bend all the same ways but there are less total rays being bent to given locations on the image so it is dimmer. This can easily be seen by looking at a lens that has only horizontal rays approaching it. All of these rays converge to the focal point; covering a portion of the lens still focuses the rays on the focal point, just less of them.