27. In the determination of focal length of a convex lens, image of grill of a window is formed on a screen. The index mark of screen holder is 37-5 cm and that of lens holder is 16-2 cm. The distance of grill from the lens is 2-5 m. Focal length of lens is
(a) 25 m
(b) 37-5 cm
(c) 162 cm
(d) 21-3 cm
Solution:
To determine the focal length of a convex lens,
The index mark of the screen holder is used for measuring the focal length and hence when the difference between the two i.e index mark of the screen holder and lens holder is subtracted, the focal length of the lens is measured, which in this case is,
37.5 cm - 16.2 cm = 21.3 cm