We have to go fetch the story in 1672 when that clever guy, Sir Isaac Newton, first published his experiments on the refraction of light. He was the one that discovered it, and just like two plus two will always be four, his principle is still as steadfast as ever. What we know is, that when light travels from one transparent substance into another of a different optical density, such as from air into water, it changes speed. The optical density is called the refractive index, which for air is 1n and for water it is 1,43n. If light strikes another substance at exactly ninety degrees (straight on), it will continue on its path without deviation. However, if it strikes the substance at an angle, the change of speed will also bring about a change of direction. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction. This change of direction is called refraction.

This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows.
The amount of bending depends on two things:

  • Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
  • Angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.

So the curve of spectacle lenses ensures that light is always striking it at an angle, except at the very middle (optic centre) of the lens, where it will pass through without bending. A higher powered lens has steeper curves, which increases the angle of incidence resulting in more bending power. As can be seen above a convex lens (a) converges light and a concave lens (b) diverges light. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retinas. We have two main curves in the eye, being the cornea and the lens.

One more thing to point out; white light is made up of all the colours of the rainbow. When a beam of white light is refracted, as can be seen with the prism below, the different colours are refracted to different degrees, causing the colour spectrum of the rainbow. This is because different colours are made up of different wave lengths.

Why do we have to wear glasses?

Refractive errors are caused, when the cornea and the crystalline lens in the eye do not have the exact curvature (bending power) in relation to the length of the eye. This means that light entering the eye is either bent too much or too little, or bent differently in one plane compared to another. The common types of refractive errors are; myopia (near-sightedness), hyperopia (far sightedness), astigmatism and presbyopia. Refractive errors are primarily caused by genetics and not by over use of the eyes.


(Close objects are clear, and distant objects are blurry)
Also known as near-sightedness, myopia is usually inherited and often discovered in childhood. Myopia often progresses throughout the teenage years when the body is growing rapidly.


It usually occurs when the front surface of the eye, the cornea, has an asymmetric curvature. Normally the cornea is smooth and equally curved in all directions, and light entering the cornea is focused equally on all planes, or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in another. This abnormality may result in vision that is much like looking into a distorted, wavy mirror. Usually, astigmatism causes blurred vision at all distances.


(close objects are more blurry than distant objects)
Also known as farsightedness, hyperopia can also be inherited. Children often have hyperopia, which may lessen in adulthood. Hyperopia can be confusing because the effect it has on vision depends on the degree, as well as the person’s age. Young people have lots of focussing ability which can focus the image back on the retina to provide clear vision at far and near. Focussing ability diminishes with age, so that older people with hyperopia may have blurred vision at far and near.


(aging of the lens in the eye)
After age forty, the lens of the eye becomes more rigid and does not flex as easily. As a result, the eye loses its focusing ability and it becomes more difficult to read at close range. This normal aging process of the lens can also be combined with myopia, hyperopia or astigmatism.

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