Question

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; (i) distant vision, and (ii) near vision?

Answer 1

Answer:

-0.18m and +0.67m.

Explanation:

Since, we know the formulae of power is 1/focal length and here in this question we know that the power is -5.5D so the focal length will be 1/-5.5 = -0.18 m.

As the focal lense is having negetive sign that means it is concave lense.

Again, power given is +1.5 D so the focal length will be 1/1.5 = +0.67 hence the positive sign means that the lense is convex.

Answer 2

→The power (P) of a lens of focal length f is given by the relation

$\large\tt\pink{Power\: (P) =\: 1/f}$

(i) Power of the lens (used for correcting distant vision) = – 5.5 D

→Focal length of the lens (f) = 1/Pf= 1/-5.5 = -0.181 m

→The focal length of the lens (for correcting distant vision) is – 0.181 m.

(ii) Power of the lens (used for correcting near vision) = +1.5 D

Focal length of the required lens (f) = 1/P

→f = 1/1.5 = +0.667 m

The focal length of the lens (for correcting near vision) is 0.667 m.