����ý

Long sight

• A long-sighted person can see objects a long distance away (they have good long sight) but can’t see objects a short distance away. A long-sighted person can read writing on a whiteboard in a classroom clearly but cannot read print in a book or newspaper sharply.
• Images of nearby objects (at 25cm from the eye) are formed behind the retina. The image is blurred.
• Long sight is due to the eyeball being too short, or the lens cannot be made thick enough by the ciliary muscles to focus the light rays on the retina. Long sight often occurs in older people as the ciliary muscles weaken with age.

Correcting for long sight

• Rays from a nearby object need to be converged more, to form the image on the retina.
• Long sight is corrected using a converging lens which starts to converge light rays from a nearby object before they enter the eye.
• Converging (convex) lenses are used in reading glasses.

Short sight

• A short-sighted person can see close objects clearly (they have good short sight), but they can’t see distant objects clearly. A short-sighted person can read a book clearly but cannot read writing on a whiteboard in a classroom or a car number plate at distance.
• The image of a distant object, say 2 m to 3 m from the eye, is formed just in front of the retina, causing it to appear blurred.
• This defect is due to the eyeball being too long or the ciliary muscles cannot make the lens thin enough.

Correcting for short sight

• Rays from a distant object need to be spread out, before they reach the lens.
• Short sight is corrected using a diverging lens which diverges the light rays from a distant object before they enter the eye.
• Diverging (concave) lenses are used in spectacles for distance viewing.

Short sight

Someone with short-sight can see near objects clearly, but cannot focus properly on distant objects.

Short sight is caused by one of the following:

• The eyeball being elongated - so that the distance between the lens and the retina is too great.
• The lens being too thick and curved - so that light is focused in front of the retina.

Short-sightedness can be corrected by placing a concave lens in front of the eye, as shown in the diagrams below.

Long-sight

Someone who is long-sighted can see distant objects clearly, but they cannot focus properly on near objects.

Long-sightedness is caused by one of the following:

• the eyeball being too short - so the distance between the lens and retina is too small
• a loss of elasticity in the lens - meaning it cannot become thick enough to focus (which is often age-related)

As a result, the lens focuses light behind the retina instead of onto it. Long-sightedness is corrected by putting a convex lens in front of the eye, as shown in the diagrams below.