Express the power (with sign) of a concave lens of focal length 20 cm.

As we know,

$\text{Power} = \dfrac{1}{\text{focal length}}$

Given,

focal length = 20 cm = 0.2m

Substituting the value of focal length in formula we get,

$\text{Power} = \dfrac{1}{\text{0.2 m}} \\[0.5em] \Rightarrow \text{Power} = 5 D \\[0.5em]$

As the lens used is concave in nature so power is negative.

Hence, Power = – 5D