The focal length of a convex lens is 20 cm. What is the power of the lens?
+5 D
+0.5 D
-5 D
-0.5 D
Correct Answer: A — +5 D
Explanation:
The power of a lens is given by P=1fP = \frac{1}{f}P=f1, where f is in metres.
For f = 20 cm = 0.20 m:
P=10.20=+5P = \frac{1}{0.20} = +5P=0.201=+5 D
