The eye with age
A grandmother read small print easily when she was young. Now small print blurs, but far signs look clear.
- A young eye sees clearly from 25 cm. That is its near point, the nearest sharp distance.
- With age, the near point moves farther away. Closer print blurs.
- A convex lens makes the 25 cm print look as far as the near point.
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Slide where you look. See which part of the lens you use.
The rule
With age, the ciliary muscles weaken and the lens gets stiffer. So the lens cannot grow thick enough for near objects. Print at 25 cm blurs. This slow change is called presbyopia. Far objects can stay clear.
With age, the near point moves farther away. A convex lens helps with reading. A bifocal lens has a concave top, for far. Its bottom is convex, for near.
A reading lens puts a 25 cm page at the near point. Its power is 4 − 100 ÷ n D. Here n is the near point in cm. The 4 D is 1 ÷ 0.25 m.
Worked example: near point 80 cm. Power = 4 − 100 ÷ 80 = 4 − 1.25 = +2.75 D. The plus sign means a convex lens.
A person who is also myopic may need a bifocal lens. Myopic means having myopia: far things blur. Example: a bifocal lens has −4 D on top. It has +2 D below.
- Top: f = 1 ÷ (−4) = −0.25 m = −25 cm. It is concave.
- Bottom: f = 1 ÷ 2 = 0.5 m = 50 cm. It is convex.