Vision (Oxford AQA IGCSE Physics)

Revision Note

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 book is placed close to the eye, which has a thick lens and hence a greater focusing power. So an image is formed from the rays converging onto the retina at the back of the eye.
When the ciliary muscles are contracted the lens becomes thicker and has a greater focusing power, so it can converge rays from nearby objects to form an image on the retina

A thicker lens can focus on objects at the far point

A tree can be seen far away from the eye, which has a thin lens and hence a smaller focusing power. So an image is formed from the rays converging onto the retina at the back of the eye.
When the ciliary muscles are relaxed the lens becomes thinner and has a smaller focusing power, so it can converge rays from far away objects to form an image on the retina

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

A book has parallel light rays entering the eye and converging to form the uncorrected image behind the retina and not on it
An eye that is long-sighted has a narrower lens with a smaller focusing power so the light rays meet and form an image behind the retina and not on it

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

A book has parallel light rays passing from it into the eye. The light rays are converged to form a corrected image on the retina at the back of the eye.
The converging lens causes the rays to converge before they reach the eye, so the corrected image is formed on the retina and not behind it
  • 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

A virtual image is formed further in front of the eye at the uncorrected near point if the object is at the near point of 25 cm in front of the convex lens. The corrected image is formed on the back of the retina.
A virtual image of the object is formed at the uncorrected near point of the eye

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

Short-sightedness occurs when parallel rays of light enter the eye and converge to form an uncorrected image in front of the retina and not on it
An eye that is short-sighted has a narrower lens with a smaller focusing power so the light rays meet and form an image behind the retina and not on it

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

Parallel light rays from distant objects pass through a concave lens before reaching the eye so the corrected image forms on the retina and not before it
The concave lens causes the rays to diverge before they reach the eye, so the image is formed on the retina and not in front of it
  • 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

Parallel rays from a far-point are diverged by a concave lens that has a focal length equal to the principal focus of the eye so an corrected image is formed on the retina.
The convex lens used must have its principal focus in the same position as the eye's faulty far point

Worked Example

The diagram shows rays of light entering a person's eye and focussed at a point. This point is not on the retina so the person sees a blurred image.

Parallel light rays pass into the eye and converge in front of the retina

a) What is the visual defect of this eye?

b) What is the cause of this defect?

c) What type of lens is used to correct this defect?

Answer:

Part a)

  • This visual defect of this eye is short-sightedness

Part b)

  • The cause of this defect is that the eyeball is too curved

OR

  • The eyeball is too long

Part c)

  • The type of lens used to correct this defect is a convex (or diverging) lens

Examiner Tip

The characteristics of long and short-sightedness can be confusing. You must learn all of this information well in preparation for answering questions in your exam.

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

A patient is lying down and covered in protective clothes. Their head is held still whilst the laser changes the shape of the lens, for IGCSE & GCSE Physics revision notes
During laser eye surgery the eye is kept open whilst the laser is used to burn the lens and change its shape.

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