Question

what is mean by power of a lens write its SI unit a student uses a lens of focal length 40 cm and another of minus 20 CM write the nature and power of each lens

Answer 1

The ability of a lens to converge/diverge a beam of light rays is expressed in terms of its Power(P).
P=1/f
The SI unit of power of lens is Dioptre.It is denoted by the letter "D"
SI unit of length is metre(m).

Hope this will help you :-)

Answer 2

power of lens is the reciprocal of the focal lens in meters.
P=1/f

f = 40 cm it is convex lens
P=1/0.4
   f = 2.5 D

f= -20 cm it is concave lens
 P=1/f
   =1/-0.2
   =-5 D


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