Question

the power of lens is the +4 diopter what will be its focal length​

Answer 1

Answer:

For example, if a lens has diopter +2, its focal length is 1000/2=500mm, and a lens of +4 diopter has focal length 1000/4 = 250mm.

Diopter, in optics, unit of magnifying power of a lens or lens system. Because the power of a lens is proportional to unity (one) divided by the focal length (see lens), the power of a lens in diopters is numerically equal to 1 m divided by the focal length in metres.

Answer 2

D = 1/F(in m)

F = 1/D

F = 1/ (+4)   m

F = +100/4   cm

F = +25 cm

so its focal length is +25cm