1.

How can you decide the focal length of convex lens to be used ?

Answer»

Solution :Here object distance (u)=-25 cm
Image distance (v) =-d (distance of near point)
Let 'F' be the focal LENGTH of bi-convex LENS.
Using the lens formula `1/f =1/v-1/u`
`implies1/f=1/(-d) -1/((-25)) implies1/f =((d-25))/(25d) IMPLIES f=(25d)/((d-25))` (f in centimetres)
If `d gt 25 cm implies` 'f' becomes positive then we NEED to use biconvex lens to correct hypermetropia.


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