The Eye

1. 1 Name [3 marks]

   Name the part of the eye that (a) refracts light as it enters, (b) controls the amount of light entering, and (c) contains the light receptors.

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   Model answer

   (a) The cornea. (b) The iris. (c) The retina.

   Mark scheme

   • Cornea — 1 mark
   • Iris — 1 mark
   • Retina — 1 mark
2. 2 Describe [3 marks]

   Describe the pathway taken by light from an object to the brain.

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   Model answer

   The light is refracted by the cornea and passes through the pupil. The lens focuses it onto the retina, where light receptors turn it into impulses. The impulses are carried to the brain by the optic nerve.

   Mark scheme

   • Refracted by the cornea and lens, focused on the retina — 1 mark
   • Receptors detect it and form impulses — 1 mark
   • Optic nerve carries them to the brain — 1 mark
3. 3 Explain [3 marks]

   Explain why the pupil gets smaller when a person goes from a dim room into bright sunlight.

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   Model answer

   Circular muscles in the iris contract and radial muscles relax, so the pupil gets smaller. This reduces the amount of light entering the eye. It protects the retina from being damaged by too much light.

   Mark scheme

   • Circular muscles contract, radial relax — 1 mark
   • Less light enters the eye — 1 mark
   • Protects the retina — 1 mark
4. 4 Compare [4 marks]

   Compare rod cells and cone cells.

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   Model answer

   Rod cells work in dim light, but they cannot detect colour, so they give a black and white image. Cone cells need bright light and detect colour, and there are three types for red, green and blue light. There are more rods than cones, and the cones are concentrated in the centre of the retina.

   Mark scheme

   • Rods work in dim light — 1 mark
   • Rods do not detect colour — 1 mark
   • Cones work in bright light — 1 mark
   • Cones detect colour (three types) — 1 mark
5. 5 Explain [4 marks]

   Explain how the shape of the lens changes when a person looks from a distant object to a near object.

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   Model answer

   The ciliary muscles contract, so the suspensory ligaments become slack. The lens is then no longer pulled flat, so it becomes thicker and more curved. This increases the amount that it bends the light, so the near object is focused on the retina.

   Mark scheme

   • Ciliary muscles contract — 1 mark
   • Suspensory ligaments slacken — 1 mark
   • Lens becomes thicker / more curved — 1 mark
   • Light is bent more so it focuses on the retina — 1 mark
6. 6 Explain [6 marks]

   Explain how the structure of the eye allows a person to see a clear image in both bright and dim light.

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   Model answer

   In bright light, the circular muscles of the iris contract, making the pupil smaller. This limits the light entering and protects the retina. The cone cells work in bright light and detect colour, giving a sharp, detailed image. In dim light, the radial muscles contract and make the pupil larger, so more light enters the eye. The rod cells are very sensitive and work in dim light, so the person can still see, although only in black and white. The lens and cornea focus the light on the retina in both cases.

   Mark scheme

   • Pupil smaller in bright light, using the iris muscles — 1 mark
   • Protects the retina — 1 mark
   • Cones work in bright light and detect colour — 1 mark
   • Pupil larger in dim light — 1 mark
   • Rods work in dim light — 1 mark
   • Cornea and lens focus the light on the retina — 1 mark

Quick check

1. 1

   Which part of the eye bends light most as it enters?

   1. AThe retina
   2. BThe iris
   3. CThe cornea
   4. DThe optic nerve
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   C: The cornea

   The cornea does most of the refraction of light as it enters the eye.

2. 2

   Which part of the eye contains the light receptors?

   1. AThe lens
   2. BThe retina
   3. CThe cornea
   4. DThe iris
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   B: The retina

   The retina contains rod and cone cells, which detect light.

3. 3

   What controls the amount of light that enters the eye?

   1. AThe retina
   2. BThe lens
   3. CThe optic nerve
   4. DThe iris
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   D: The iris

   The iris changes the size of the pupil, which controls how much light enters.

4. 4

   What happens to the pupil in bright light?

   1. AIt gets smaller
   2. BIt gets bigger
   3. CIt stays the same
   4. DIt closes completely
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   A: It gets smaller

   The pupil gets smaller, to stop too much light reaching the retina.

5. 5

   Which cells detect colour?

   1. ARod cells
   2. BCone cells
   3. CCornea cells
   4. DIris cells
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   B: Cone cells

   Cone cells detect colour, and there are three types, sensitive to red, green and blue.

6. 6

   Why can we only see in black and white in very dim light?

   1. ACone cells do not exist at night
   2. BThe pupil gets too small
   3. COnly the rod cells work
   4. DThe lens becomes thinner
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   C: Only the rod cells work

   In very dim light only the rod cells work, and rods cannot detect colour.

7. 7

   What carries impulses from the retina to the brain?

   1. AThe optic nerve
   2. BThe spinal cord
   3. CThe motor neurone
   4. DThe cornea
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   A: The optic nerve

   The optic nerve is a bundle of sensory neurones that links the retina with the brain.

8. 8

   To look at a near object, what happens to the lens?

   1. AIt gets thinner
   2. BIt stays flat
   3. CIt moves backwards
   4. DIt becomes thicker and more curved
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   D: It becomes thicker and more curved

   The lens becomes thicker and more curved, because the ciliary muscles contract and the ligaments slacken.

9. 9

   Which muscles contract to make the pupil larger in dim light?

   1. ACircular muscles
   2. BRadial muscles
   3. CCiliary muscles
   4. DMuscles of the cornea
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   B: Radial muscles

   The radial muscles contract to pull the iris open and make the pupil larger. The circular muscles relax.