Here we have a concave lens. Now, the focal length of a concave lens is considered negative, so it is to be written with a minus sign.
Thus, Focal length, f=`-50` cm
`=-50/100` m
`=-0.5` m
Now, Power, P`=(1/("f in metres))`
`P=1/-0.5`
`P=(-1 x 10)/(5)`
`P=-2` dioptres (or `-2`D)
Thus, the power of this concave lens is, `-2` dioptres which can also be written as, `-2`D. The minus sign with the power indicates that it is diverging lens or concave lens.
