Question

Define power of a lens.
(ii) A child is using a spectacle with power of –2.5 D. What is meant by the
negative sign?
(iii) Find the focal length of the lens used.

Answer 1

i)Power of lens : 
The power of lens is the degree of convergence or divergence of rays falling on it.
It can be defined as reciprocal of focal length.
P=1/f (in m)

2) A child is using -2.5D

here negative sign indicates it is concave lens and child is suffering from myopia disease  where can see near things clearly but finds difficulty in seeing far off things.

3) P=-2.5D
-2.5D=1/f
f= -1/2.5
=-10/25
= - 2/5
=- 0.4m 
= -0.4 x 100
=-40cm

∴Focal length of given concave lens is -40cm

Answer 2

Answer:

this will be the answer of 3 bit