Question

An opaque card is placed over an imaging lens such that it covers the top half of the lens. An image being formed by that lens is seen on a screen at some distance. As the card is put in place in front of the lens the image shows

a) The image being blocked on the bottom half

b) No answer provided

c) Only a change in brightness and resolution, nut no change in image coverage

d) The image being blocked on the top half

2. In order for a normal eye to see things at a great distance the focal length of the lens must be equal to

a) One half the distance from the lens to the retina

b) The image or in other words the distance from the lens to the retina

c) The interocular distance between the pupil and the uvea

d) None of the above

Answer 1

Hey there!

(1) If one half of the lens is covered , what really happens is that the intensity of the image , that is the brightness of the image decreases also with it's resolution. There wouldn't be any change in the image coverage. In other words, the whole image could be seen through one half of the lens with less clarity. Hence OPTION C.

(2)To view an object which is present at infinity , the lens should be at the focal point to view the image. In that sense here the distance between the lens and the retina should be equal to the eye lens and the cornea. This happens because the eye muscles would be relaxed at this point of time. The focal length of the human eye is about 17 to 20 mm in average.Hence the right option is OPTION B.

I hope the solution helps.. Feel free to comment and discuss further... Cheers :)