5.21 Eyes 2 - Accommodation & Visual Defects

Question 1

What is accommodation in terms of the eye?

Answer 1

A reflex that changes the refractive power of the lens.

Question 2

If the object an eye is looking at is far away, will the light rays be almost parallel or have a wide angle?

Answer 2

Almost parallel

Question 3

If the object an eye is looking at is close, will the light rays be almost parallel or have a wide angle?

Answer 3

Wide angle

Question 4

Describe how the iris changes shape to focus on near objects

Answer 4

The ciliary muscle contracts inwards towards the lens. As it is now closer to the lens, the suspensory ligaments slacken, and because they are no longer pulling tight on the lens, the lens is free to return to its natural fatter shape so can now refract light more strongly.

Question 5

Describe how the iris changes shape to focus on distant objects

Answer 5

For distant objects, light doesn’t need to be refracted as strongly which means that the lens doesn’t need to do as much because the cornea has already refracted the light the most of the way. To reduce the refractive power of the lens, it needs to be stretched out so that its surface is less curved. In order to achieve this, the ciliary muscle relaxes, moving away from the lens and pulling the suspensory ligaments tight which stretches the lens out.

Question 6

Describe how long-sightedness works

Answer 6

Long-sighted people are unable to focus on near objects:
- This occurs when the lens is the wrong shape and doesn’t refract (bend) the light enough or the eyeball is too short.
- Light rays don’t converge (meet at a point) on the retina - instead images of near objects are brought into focus behind the retina.

Question 7

How do we combat long-sightedness?

Answer 7

You can use glasses or contact lenses with a convex lens to correct it. A convex lens curves outwards so it’s fattest in the centre. The lens refracts light so it starts to converge before it enters the eye. The increased convergence allows the image to be brought into focus on the retina.

Question 8

Why is long-sightedness more common in older people?

Answer 8

As you get older, your eye’s lens loses flexibility, so it can’t easily spring back into shape. This means light can’t be focused well for near viewing - so older people are often long-sighted.

Question 9

Describe how short-sightedness works

Answer 9

Short-sighted people are unable to focus on distant objects:
- This occurs when the lens is the wrong shape and refracts the light too much or the eyeball is too long.
- Light rays converge before they reach the retina, so images of distant objects are brought into focus in front of the retina.

Question 10

How do we combat short-sightedness?

Answer 10

You can use glasses or contact lenses with a concave lens to correct it. A concave lens curves inwards, so that it’s thinner in the centre than at the edge. The lens refracts light so that light rays diverge (move apart) before they enter the eye. The reduction in convergence allows the image to be brought into focus on the retina.

Question 11

What is the medical term for long-sightedness?

Answer 11

Hyperopia

Question 12

What is the medical term for short-sightedness?

Answer 12

Myopia

Question 13

Which two structures refract (bend) light entering the eye?

Answer 13

• Lens
• Cornea

Question 14

When light passes into the eye, where on the retina should the light be focused on?

Answer 14

The fovea