Answers · The eye

The eye — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. ER1 · Return

Model answer. (a) The receptors in the eye respond to light. (b) The receptors in the nose respond to chemicals in the air.

A full answer includes

(a) I wrote light.
(b) I wrote chemicals.
(b) I did not write smells or air.

Also accept

(a) light; (b) chemicals

Watch for

“(b) Smells.” — A smell is made by chemicals in the air: chemicals are the stimulus the nose responds to.

2. ER2 · Return

Model answer. (a) The lens focuses light on to the retina. (b) The retina contains the light receptors, some of them sensitive to light of different colours.

A full answer includes

(a) I wrote that the lens focuses light on to the retina.
(b) I wrote that the retina contains light receptors.
(b) I wrote that some of the receptors are sensitive to light of different colours.
(a) I did not write that the lens controls how much light enters.

Also accept

(a) focuses light on to the retina; (b) contains light receptors, some sensitive to different colours

Watch for

“(a) It controls how much light enters.” — That is the iris. The lens focuses the light on to the retina.

3. ER3 · Return

Model answer. The blind spot, where the optic nerve leaves the eye, has no light receptors. Everywhere else the retina has receptors.

A full answer includes

I chose: where the optic nerve leaves the eye.
I did not choose "nowhere": every eye has a blind spot.

Also accept

where the optic nerve leaves (the blind spot)

Watch for

“Where the image of what you look at falls.” — Light from what you look at falls on receptors and is seen. The place with none is where the optic nerve leaves.
“At the front edge, closest to the iris.” — The edges of the retina still have receptors. The place with none is where the nerve fibres gather and leave.
“Nowhere: the whole retina has receptors.” — There is one place with none: the blind spot, where the optic nerve leaves.

4. PR1 · Return

Model answer. (a) As the lamp setting went down, the pupil diameter increased: the pupil widened. (b) More light enters the eye, so enough light reaches the receptors in the retina to see in the dim light.

A full answer includes

(a) I wrote that the diameter increases: the pupil widens.
(b) I wrote that more light enters the eye.
(b) I wrote that enough light then reaches the retina to see.
(a) I did not write that the diameter decreases.

Also accept

(a) increases; (b) more light enters, enough reaches the retina

Watch for

“(a) It decreases.” — The pupil narrows in bright light. In dim light it widens.

5. PR2 · Return

Model answer. The circular muscles form rings round the pupil: when they contract, the pupil narrows. The radial muscles run outwards: when they contract, the pupil widens.

A full answer includes

(a) I wrote the circular muscles, which narrow the pupil when they contract.
(b) I wrote the radial muscles, which widen the pupil when they contract.
I did not write the ciliary muscles.

Also accept

circular, narrows; radial, widens

Watch for

“Ciliary muscles.” — The ciliary muscles change the shape of the lens. The iris muscles are circular and radial.

6. PR3 · Return

Model answer. The stimulus is a change in light intensity, detected by light receptors in the retina. The iris muscles are the effector, and the change in pupil diameter is the response.

A full answer includes

I chose: a change in light intensity.
I did not choose the contraction of the iris muscles, which is the effector at work.

Also accept

a change in light intensity

Watch for

“The contraction of the iris muscles.” — The iris muscles are the effector, and their action changes the pupil diameter, which is the response. The stimulus is the change in light intensity.
“A change in the distance of the object.” — A change in distance starts accommodation, which changes the shape of the lens. The pupil reflex responds to the light.
“Impulses in the optic nerve.” — Impulses carry the message after the receptors have detected the stimulus. The stimulus is the change that is detected.

7. FR1 · Return

Model answer. (a) The ciliary muscles relax. (b) The suspensory ligaments are pulled tight. (c) The lens is pulled thinner, so it refracts light less.

A full answer includes

(a) I wrote that they relax.
(b) I wrote that they are pulled tight.
(c) I wrote that the lens becomes thinner.
(a) I did not write that the ciliary muscles contract.

Also accept

(a) relax; (b) pulled tight; (c) thinner

Watch for

“(a) They contract.” — The ciliary muscles contract for near objects. For distant objects they relax.

8. FR2 · Return

Model answer. (a) The fovea or the blind spot: neither has any rods. (b) A sample from the fovea would be packed with cones; a sample from the blind spot, where the optic nerve leaves, would have no cones at all.

A full answer includes

(a) I wrote the fovea.
(a) I wrote the blind spot.
(b) I wrote that the fovea is packed with cones.
(b) I wrote that the blind spot has no cones.
(a) I did not write the edge of the retina.

Also accept

(a) fovea, blind spot; (b) many cones means fovea, none means blind spot

Watch for

“(a) The edge of the retina.” — The edge has rods, though fewer than closer in. Only the fovea and the blind spot have none.

9. FR3 · Return

Model answer. Rods are more sensitive to light than cones, so they give night vision; cones need brighter light. Cones give colour vision, with three kinds each absorbing light of a different colour; rods give no colour vision.

A full answer includes

I wrote that rods are more sensitive to light than cones.
I wrote that rods give night vision.
I wrote that cones give colour vision and rods do not.
I wrote that there are three kinds of cone, absorbing light of different colours.
I did not write that rods see in colour.

Also accept

rods more sensitive / night vision; cones colour vision, three kinds

Watch for

“Rods see in colour at night.” — Rods work at night but give no colour. Colour vision comes from cones.

10. FR4 · Return

Model answer. (a) The fovea is on the retina at the back of the eye, on the line of sight. (b) It gives the most detailed vision: it is packed with cones.

A full answer includes

(a) I wrote that it is on the retina at the back of the eye.
(a) I wrote that it is on the line of sight.
(b) I wrote that it gives the most detailed vision.
(a) I did not write where the optic nerve leaves.

Also accept

(a) back of the retina on the line of sight; (b) most detailed vision

Watch for

“(a) Where the optic nerve leaves.” — That is the blind spot, which has no receptors.