Question

if the power of A lens is 5 dioptres find its focal length

Answer 1

P = 5D

f =?

Therefore, P = 1/f

So, f = 1/P

f = 1/5

f = 0.2 m

Answer 2

power=1/focal length
therefore focal length=1/power
f=1/5 D
f=0.2 m=20 cm