Question

The nearest distance of distinct vision for the grandpa who is suffering from long-sightedness is 75 cm. After wearing the spectacles prescribed to him, this distance is reduced by 50 cm. What is the power of the spectacles?

Answer 1

We need to find the power of the lens that enable him to read a book that is held at 25 cm from the eye.

→ v (far point) = -75 cm

u (near point) = -25 cm

P = ?

Both u and v are negative, because the object and the image are on the left side of the lens

→ Applying lens formula to obtain the focal length of the lens

1/f = 1/v - 1/u

= (1/ -75) - (1/-25)

= (1/-75) + 1/25

= (-1+3)/75

= 2/75

f = +75/2 = 37.5 cm = 0.375 m

→ Obtaining power of lens:

P = 1/f

= 1/0.375

= +2.66 D

• Therefore, a convex corrective lens used to treat the eye defect (as the focal length is positive)
• And the power of the lens is +2.66 D