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What is myopia?
What is hypermetropia?
What is astigmatism?
Why do people need reading glasses after 40?
Why do some people do not need reading glasses after 40?
Why do some people need reading glasses before 40 ?
What harm do adults get if they do not wear their glasses ?

Myopia

A normal eyeball has an average length of 24 mm. A parallel beam of light rays come to focus at the retina and they do not need glasses. Some people , on the other hand are born with somewhat longer eyeballs. In such eyes a parallel beam of light comes to focus in front of the eye.They need to have a lens with thin center ( concave lens) in front of their eyes to focus the light rays into the retina.This called myopia. When we say that someone has -3 D myopia , we mean that he would need a concave glass of 3 diopteric strength to see clearly .In colloquial terms this is also called "short sightedness"
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Hypermetropia

In hypermetropia the eyeballs are smaller than normal. Therefore a parallel beam of light comes to focus behind the retina and they need a lens with thick center ( convex lens) in front of their eyes to focus light on to retina and see clearly.

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Astigmatism

In astgmatism the eyeball in not sperical but oblong. It might be long in one axis and normal in the other . Somewhat  like in the orange for example.They need appropriate power of lenses only in that axis.  In the optical prescription this is written in degrees following the power of the lens.

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Reading glasses after 40

All eyes need a lens power of +3.0 Diopters for reading and doing close work. In the younger person the cilliary body muscle inside the eye contracts and  helps the eye  to get an additional +3.0 D power . This is by a physiological process called accommodation. After the age of 35 this muscle begins to weaken and people need reading glasses. The rule of the thumb for reading glasses is:
 
35 yrs - 40 yrs    +0.5Dsph
40 yrs - 45 yrs    +1.0 Dsph
45yrs  -  50 yrs   + 1.5 Dsph
50 yrs - 55 yrs    + 2.0 Dsph
55 yrs -  60 yrs   + 2.5 Dsph
> 60  years          + 3.0 Dsph.

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Who doesn't need a reading glass?

The reading glasses add algebrecally to what have power you have for distance. So a myope of -2.0 Dsph would not need reading glasses till 50 years and high myopes i.e. - 5.0 Dsph can all throughout their life read withought needing reading glasses.
 
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Who needs a reading glass before reaching 40 ?

Hypermetropes i.e those already having a plus power for distance are overburdened when they approach 40. A hypermetrope of +2.0 Dsp might need reading glasses by early thirties.

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What harm do adults get if they do not wear their glasses?

If you have been prescribed glasses but are not wearing them, then you could get one of two things. You either get a headache or you just cannot see. You would not harm your eyes or damage some delicate structures inside the eye by not wearing your glasses. This is ,however,
not so wioth children. Children bellow ten years must use their glasses at all time for helping their vision to develop .Ele they would be left with lazy eyes.

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