A lens of focal length 90 mm is used as a magnifier.The object being viewed is 9.9 mm long,and is positioned at the focal point of the lens.The angle subtended by the image at infinity,in milliradians,is closest to:
A)110
B)73
C)37
D)147
E)183

Question

Quizplus · Accepted Answer

The magnification produced by the lens is given by:
M = -(s'/s)
where s' is the image distance and s is the object distance.
In our case, s' is infinity (since the image is formed at infinity) and s is equal to the focal length (since the object is placed at the focal point).
Therefore, M = -(infinity/90) = 0
This implies that the image formed is the same size as the object.
Now, the angle subtended by the object at the lens is given by:
θ = tan^-1 (h/s)
where h is the height of the object (which is equal to its length in this case).
Thus, θ = tan^-1 (9.9/90) = 0.11 rad = 110 mrad (rounded to the nearest whole number).
Therefore, the answer is (A) 110.

A Lens of Focal Length 90 Mm Is Used as a Magnifier.The