A lens of focal length 90 mm is used as a magnifier by a person with a near point of 25 cm.The object being viewed is 9.9 mm long,and is positioned at the focal point of the lens.What is the angle subtended by the image at infinity,in milliradians?
A)110 mrad
B)73 mrad
C)37 mrad
D)147 mrad
E)183 mrad

Question

Quizplus · Accepted Answer

When an object is placed at the focal point of a lens, the image formed is at infinity, and the angle subtended by the image can be calculated using the formula for angular size, $\theta = \frac{size}{focal\ length}$. Here, the size of the object is 9.9 mm and the focal length is 90 mm. Converting the size to meters, we get 0.0099 m. Thus, $\theta = \frac{0.0099}{0.09} = 0.11$ radians, which is 110 milliradians (mrad) when converted from radians (1 rad = 1000 mrad).

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