A person needs a lens of power - 5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptres. What is the focal length of the lens required for correcting (1) distant vision and (2) near vision ?

Explanation

For distant vision, f= ?,
`P=-5.5, D=-5.5 m^(-1)`
Now, `f = (1)/(P)`
`therefore f=(1)/(-5.5 m^(-1))`
= -0.182 m
= -18.2 cm (concave lens)
(2) For near vision, f= ?,
` P= + 1.5, D = + 1.5 m^(-1)`
Now, `f = (1)/(P)`
`therefore f= (1)/(+1.5 m^(-1))`
= 0.667 m
= 66.7 cm (Convex lens)