Vision (Oxford AQA IGCSE Physics)

Range of Vision

• Our eyes contain convex lenses and a cornea that focus light onto the retina at the back of the eye

• A normal human eye can see from 25 cm in front of itself to infinity

  • So it can clearly see objects more than 25 cm away

• The near point of a normal eye is 25 cm

• The far point of a normal eye is infinity

• The distance between these points is the range of vision

A thicker lens can focus on objects at the near point

A thicker lens can focus on objects at the far point

Correcting Vision with Lenses

Long-sightedness

• Long-sightedness happens when the lens is less curved than normal or the eyeball is too short

  • So the cornea and lens are too weak

• This means light rays are not refracted enough, so the uncorrected image falls behind the retina (rather than on it)

  • Close objects appear blurry because people with long-sightedness are unable to focus on near objects

• Remember that a "normal-sighted" person has a near point at around 25 cm 

  • A long-sighted person, however, has a near point which is further than this

Ray diagram of long-sightedness

Treatment of long-sightedness

• A long-sighted person may need to wear glasses for reading but might remove them when driving because they can easily focus on objects far away from the eye

• Long-sightedness can be corrected using contact lenses or glasses with a convex lens

  • The convex lens adds to the refraction of light by the eye's cornea and lens

  • The lens causes the light rays to form a corrected image on the retina (and not behind it)

The effect of a convex lens on a long-sighted eye

• Choosing the correct convex lens depends upon the position of the near point for a person

• The lens must first produce a virtual image of objects that are 25 cm away at the eyes uncorrected near point

  • Before bringing the rays to a focus to form the corrected image on the retina

A ray diagram showing the uncorrected near point of an object at 25 cm

Short-sightedness

• Short-sightedness happens when the lens is more curved than normal or too long

  • So the cornea and lens are too powerful

• This means light rays are refracted too much so the uncorrected image falls in front of the retina (rather than on it)

  • Distant objects appear blurry as people with short-sightedness cannot focus on objects that are far away

• Remember that a "normal-sighted" person has a far point at infinity

  • A short-sighted person, however, has a far point that is less than infinity

Ray diagram of short-sightedness

Treatment of short-sightedness

• A short-sighted person can remove their glasses if they wish to read or view something close-up because it is easy for them to focus on objects close to the eye

• Short-sightedness can be corrected using contact lenses or glasses with a concave lens

  • The concave lens reduces the refraction of the light caused by the eye's cornea and lens

  • So the light rays diverge before reaching the eye

  • So a corrected image now forms on the retina (and not in front of it)

The effect of a concave lens on a short-sighted eye

• So choosing the correct concave lens depends on the position of the far point for a person

• The lens used has its principal focus in the same position as the eye's faulty far point

  • Before bringing the rays to a focus to form the corrected image on the retina

A ray diagram showing the far point of the eye and the focal length of the lens

Correcting Vision with Lasers

• Lasers can be used in eye surgery to more permanently correct visual defects

• Following laser eye surgery someone with a visual defect may no longer have the defect or may have a less severe defect

  • Meaning they no longer need to wear corrective lenses or may be able to wear less powerful lenses

• A laser produces a narrow concentrated beam of light

• In medicine, lasers are often used for cutting tissue and sealing blood vessels (called cauterising)

• In laser eye surgery lasers are used to change the shape of the lens of the eye

  • This corrects the vision defect caused by a lens which is too thin or too thick

Laser eye surgery