Question

define power of a lens what is the unit one student uses a lens of focal length 15 cm and another of - 50 cm what is the nature of the lens and it is our used by which of them​

Answer 1

Answer:

power of a lens

the power of a lens is the reciprocal of the focal length .it is represented by the letter P

Explanation:

The SI unit of power of a lens is DIOPTRE. It is denoted by thr letter D

power of a lens( P) = 1/focal length of the lens (in m)

• The power of a lens is inversely proportional to its focal length .

A lens of short focal length has more power where as lens of a long focal length has less power

• A convex lens has a positive focal length and a convex lens has a negative focal length.
• DIOPTREMETER an instrument for measuring the power of a lens .

in case 1

focal length (f) =40cm =40/100 =2/5 =0.4m (1cm =1/100m)

P =1/f

P = 1/-0.2P = - 10/2 = - 5D

P = - 5D

Lens of power +2.5D(40cm focal length) is converging (convex ) lens of power -5D -(-20cm focal length ) is diverging (concave)