Question

The power of lens is 2.5 D. What is its focal length ?​

Answer 1

The power of lens is given by :

P=1/f

2.5=1/f

f=1/2.5

f= 0.4 m

Answer 2

Answer:

1. Power P =1/f = 2.5D

f= 1/p= 100/2.5= 40 cm

Here f= 40 cm which is positive , it means the lens is a convex lense .

2. In case of a concave f is negative . So in the question f = 30 cm

Magnification m = -v/u =3

It means v=-3u

Mirror formula =1/f =1/v+1/u

1/(-30)= 1/(-3u)+1/u

-1/30 =1/u-1/3u

-1/30=3/(3u)

u=-20cms

It means the object is placed 20 cms in front of the mirror. Since focal length is -30 cm , it means the object is placed between focus and optical lenses .