Question

a person needs a lens of power + 2.5 D for correcting his vision .name the defect of vision he is suffering from .which type of lens will he be using for correcting the defect? also find the focal length of the lens

Answer 1

Power of lens = +2.5D , e.g., positive means lens is converging.
A converging lens is convex lens . Hence person is using a convex lens for correcting his vision.
Convex lens is used to correct myopia . A myopic eye, can't able to see distinct object clearly but can see nearby object clearly . the reason of this defect is elongation of eyeball and excessive curvature of the lens.

Now, P = +2.5D
P = 1/f
⇒f = 1/P = 1/2.5 m = 100/2.5 = 40cm

Hence , focal length of convex lens is 40cm

Answer 2

If the power nd focal length is +ve then the defect is hypermetropia and if power nd focal length is -ve then the defect is myopia.

Here+2.5 is power .. so hypermetropia is the defect in the question ... It can be removed by convex lens..

P= 1/f

2.5= 1/f

F = 0.4 m

F=40 cm