Question

(a) The focal length of a convex lens is 25 cm. Express its power with sign. $\left[2\text{marks}\right]$

(b) The power of a lens is -2.0 D. Find its focal length and its kind. $\left[2\text{marks}\right]$

(c) What is a magnifying glass ? State its two uses. $\left[2\text{marks}\right]$

Answer 1

(a) P=1/F=1/+0.25
P=+4D $\left[2\text{marks}\right]$


(b) F=1/P
=1/-2
=-0.5 m or -50 cm
power is negative so it is a concave lens. $\left[2\text{marks}\right]$

(c) Magnifying glass is a convex lens of short focal length. It is mounted in a lens holder for practical use.
It is used to see and read the small letters and figures.
It is used by watch makers to see the small parts and screws of the watch. $\left[2\text{marks}\right]$