Light that's reflected from objects all over passes through the eye lens and forms images on the retina at the back of the eyeball. This activates the nerves and your brain develops the moving photos so formed.

Our eye lens is convex, similar in proportions to an aspirin pill. And this lens has the feature of twisting light rays that go through it to create an inverted picture of the thing originating the rays.

We can illustrate this with a magnifying glass. Rather like two saucers placed rim to rim, the magnifying glass is thicker in the middle than at the edges. At certain distances this lens can be used to give an enlarged view of something, because of the way it bends light rays passing through it. Magnification, however, will occur only when the object being viewed is held close to the lens, that is, closer than twice the focal length of the lens. But if you increase the distance between your eye and the magnifying glass, you will notice that everything appears upside down. Why? Because of the bending inward of the light rays as they pass through the glass. The image is inverted.

Light rays that pass through the center of a convex lens do not bend or refract to any noticeable degree. But those striking the lens at a distance from the center are refracted to pass through a definite point called the focal point. The distance between this point and the center of the lens is called the focal length.

The other type of lens, called concave, is shaped like two saucers positioned base to base, being thicker at the edges than in the centre. This zoom lens diverges or spreads light rays that go through it. Concave lenses are normally used in mixture with convex lenses, and their capability to spread light rays offers been adapted as an help to eyesight.

The Lenses for Glasses

The eye lens is flexible and may be bent, pulled away or elongated and squashed flatter. It allows quick, accurate concentrating without distortion. However, growing older can harden the eye lens or the attached ciliary muscles, making adjusting and clear focusing more challenging.

Your eye lens is at rest when you view distant objects, but is squeezed thicker by the ciliary muscles to focus on things nearby. Because of this muscular activity we get eyestrain when doing work close to the eyes or reading or writing.

If the eyeball is too long, the image is focused short of the retina and appears blurred, causing nearsightedness. This can be corrected by using glasses of the concave variety, which diverge the light entering the eye and help the eye’s convex lens to focus on the retina properly.

On the other hand, farsightedness occurs because the eyeball is too small and the image is formed behind the retina. The formation of an image behind the retina also occurs when the eye’s lens loses its power of accommodation and reaches a point where it cannot assume the deeply curved shape necessary for focusing on nearby objects. Usually affecting those passing middle age, this condition is known as presbyopia, requiring convex segments in the spectacle to correct the weakness.