Express the power (with sign) of a concave lens of focal length 20 cm.
As we know,
Power=1focal length\text{Power} = \dfrac{1}{\text{focal length}}Power=focal length1
Given,
focal length = 20 cm = 0.2m
Substituting the value of focal length in formula we get,
Power=10.2 m⇒Power=5D\text{Power} = \dfrac{1}{\text{0.2 m}} \\[0.5em] \Rightarrow \text{Power} = 5 D \\[0.5em]Power=0.2 m1⇒Power=5D
As the lens used is concave in nature so power is negative.
Hence, Power = – 5D
