A myopic person has been using spectacles of power –1.0 dioptre for distant vision. During old age he also needs to use separate reading glass of power + 2.0 dioptres. Explain what may have happened.
The myopic person uses a spectacle of power, P = -1.0 D
Focal length of the given spectacles, f = 1/P
f = 1/(-1 × 10-2 )
f = -100 cm
Hence, 100 cm is the far point of the person. He might have a normal near point of 25 cm. The objects placed at infinity produce virtual images at 100 cm, when spectacles are being used. He is able to see the objects placed between 100 cm and 25 cm when he uses the ability of accommodation of the eye-lens.
During old age, the person uses reading glasses of power, P’ = + 2 D
Old age decreases the ability of accommodation. This defect is called presbyopia. As a result, he is unable to see clearly the objects placed at 25 cm.