Question

Define power of lens ? writes it s.i unit .a student uses focal length 40 cm and -20cm write nature and power of lens . also find power and nature of. Combination of this two lens?

Answer 1

power of lens is the reciprocal of its focal length
SI unit of power is dioptre(D)
When f is 40 them it is convex lens.
power is 2.5 D
when f is --20 then it is concave lens.
power is -5 D
power of combination of lens is 2.5-5= -2.5D
it will be called a bifocal lens.