5.19 - The Eye - Accommodation

Accommodation - Focusing near and distant objects

• Accommodation is the process of changing the shape of the lens to focus light onto the retina.
• The lens sits just behind the pupil, held by a ring of suspensory ligaments attached to the ring of ciliary muscle around the inside of the eye.
• Contracting or relaxing the ciliary muscle changes the tension in the suspensory ligaments, which changes how rounded the lens is:
  • A fatter, more curved lens refracts (bends) light more strongly.
  • A thinner, flatter lens refracts it less.
• Most of the bending of light happens at the cornea at the front of the eye. The lens fine-tunes the focus so the light meets exactly on the retina.

Focusing on a near object

Light entering the eye is spreading out, so it needs to be bent a lot.

1. The ciliary muscle contracts.
2. This slackens the suspensory ligaments, taking the tension off the lens.
3. Freed from that pull, the lens springs into a fatter, more rounded shape.
4. It refracts the light by a larger amount, bringing the near object into focus on the retina.

Focusing on a distant object

Light entering the eye is almost parallel, so it needs less bending.

1. The ciliary muscle relaxes.
2. The suspensory ligaments pull tight.
3. This tension stretches the lens into a thinner, flatter shape.
4. It refracts the light by a smaller amount, just enough to bring the distant object into focus on the retina.

Short-sightedness and long-sightedness

• Some people can't focus light correctly onto the retina, giving a blurred image.
• There are two common defects - one affects near vision, the other affects distant vision.
• Each is corrected by placing a lens of the right shape in front of the eye.

Long-sightedness (hyperopia)

• The person can't focus clearly on near objects.
• Caused by either:
  • The lens being the wrong shape and not refracting the light enough, or
  • The eyeball being too short.
• Light from a near object is focused behind the retina, so the image is blurred.
• Corrected with a convex lens - curves outwards, fattest in the middle.
  • It starts to refract the light so it's already converging before entering the eye, bringing the image forward onto the retina.

Short-sightedness (myopia)

• The person can't focus clearly on distant objects.
• Caused by either:
  • The lens being the wrong shape and refracting the light too much, or
  • The eyeball being too long.
• Light from a distant object is focused in front of the retina, so the image is blurred.
• Corrected with a concave lens - curves inwards, thinner in the middle than at the edges.
  • It refracts the light so the rays diverge (spread apart) a little before entering the eye, reducing the convergence and letting the image fall back onto the retina.

Other ways to correct vision defects

• Contact lenses - Sit on the surface of the eye, shaped the same way to compensate for the fault.
• Laser eye surgery - A laser reshapes the cornea so it refracts light by the right amount.
• Replacement-lens surgery - The natural lens is replaced with an artificial one. You are expected to be able to read, explain, and complete ray diagrams showing where the image forms and how a lens corrects it.

Cataracts and colour blindness

As well as focusing defects, the lens and the light receptors can develop other faults that affect vision.

Cataracts

• A cataract is a cloudy patch that forms on the lens of the eye.
• The cloudiness blocks some of the light passing through the lens, so less light reaches the retina and a person with a cataract has:
  • Blurred vision
  • Colours that look less vivid
  • Difficulty seeing in dim light, and dazzle from bright lights
• Cataracts often develop in older people, because the tissues of the eye degrade over time.

Treatment:

• An operation removes the faulty lens and replaces it with an artificial lens made of clear plastic.
• Like any surgery, this carries a small risk of complications, such as damage to the retina.

Colour blindness

• Colour blindness is a difficulty in telling certain colours apart.
• The most common form is red-green colour blindness, where a person cannot easily distinguish reds from greens.
• It happens when the red cones or green cones in the retina do not work properly, so the eye cannot compare the signals from the different cone types in the normal way.
• There is no cure, because the faulty cone cells cannot be replaced or repaired.
• Tinted lenses can sometimes help a colour-blind person tell some colours apart more easily.