Question

express the power of a concave lens of focal length 20cm with its sign

Answer 1

Focal length of concave lens, F = -20 cm = -0.2m
Power of lens = 1/F
          = 1/(-0.2)
          = -5D

Answer 2

Given,
f = -20 cm → (-20)÷100 →(- 0.2m)

p = 1÷(-0.2)

→ P = 1×10 ÷(-2)

→ P = -5D
HOPE ITS HELPFUL