Answers · The eye

The eye — final check key

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

1. EK1 · Final check

Model answer. Start where the light enters, on the right: C is the cornea, E the iris (its two halves) and G the pupil, the gap between them; A is the lens behind it. B is the retina lining the back, D the optic nerve leaving the eye and F the blind spot, where the nerve leaves and there are no light receptors (rods or cones).

A full answer includes

I wrote A lens.
I wrote B retina.
I wrote C cornea.
I wrote D optic nerve.
I wrote E iris.
I wrote F blind spot.
I wrote G pupil.
I did not swap E and G: the halves are the iris, the gap is the pupil.

Also accept

A lens; B retina; C cornea; D optic nerve; E iris; F blind spot; G pupil

Watch for

“G is the iris.” — G marks the gap between the two halves of the coloured ring: the gap is the pupil, the halves are the iris.

2. EK2 · Final check

Model answer. (a) The cornea refracts (bends) light as it enters the eye. (b) The iris controls how much light enters the pupil. (c) The optic nerve carries impulses from the retina to the brain.

A full answer includes

(a) I wrote that the cornea refracts light.
(b) I wrote that the iris controls how much light enters the pupil.
(c) I wrote that the optic nerve carries impulses to the brain.
(a) I did not write that the cornea focuses light on to the retina.
(c) I did not write that the optic nerve carries light.

Also accept

(a) refracts light; (b) controls how much light enters the pupil; (c) carries impulses to the brain

Watch for

“(a) It focuses the light on to the retina.” — The lens does the focusing on to the retina. The function of the cornea is to refract (bend) light as it enters the eye.

3. EK3 · Final check

Model answer. (a) The ear detects the thunder: its receptors respond to sound. (b) The skin detects the warmth: its receptors respond to temperature.

A full answer includes

(a) I wrote the ear.
(a) I wrote sound.
(b) I wrote the skin.
(b) I wrote temperature.
(b) I did not write the eye or light.

Also accept

(a) ear, sound; (b) skin, temperature

Watch for

“(b) The eye; light.” — You see the sun with your eyes, but its warmth is felt by receptors in the skin: the stimulus is temperature.

4. PK1 · Final check

Model answer. (a) Q, then P, then R. (b) Q has the widest pupil, 7.5 mm, and R the narrowest, 3.0 mm. The dimmer the light, the wider the pupil reflex makes the pupil, so that more light enters; in brighter light it narrows the pupil and less light enters.

A full answer includes

(a) I wrote Q, P, R.
(b) I compared the pupil diameters.
(b) I wrote that the dimmer the light, the wider the pupil.
(a) I did not put the widest pupil in the brightest light.

Also accept

(a) Q, P, R; (b) the dimmer the light, the wider the pupil

Watch for

“(a) R, P, Q.” — That is the order from the brightest to the dimmest. The widest pupil, Q, goes with the dimmest light.

5. PK2 · Final check

Model answer. The stimulus is the decrease in light intensity. Light receptors in the retina detect it, and impulses pass to the brain and out to the effector, the muscles of the iris. The response is that the pupil widens, so more light enters and the person can see better in the dark. It happens automatically, without a decision.

A full answer includes

I wrote that the stimulus is the decrease in light intensity.
I wrote that light receptors in the retina detect it.
I wrote that the effector is the muscles of the iris.
I wrote that the response is the pupil widening.
I wrote that more light then enters the eye.
I wrote that it happens automatically, without a decision.
I did not name the lens or the retina as the effector.

Also accept

stimulus less light; receptors in retina; effector iris muscles; response pupil widens; more light enters; automatic

Watch for

“The effector is the lens.” — The lens focuses light. In the pupil reflex the effector is the muscles of the iris.

6. PK3 · Final check

Model answer. In bright light the circular muscles contract and the radial muscles relax, so the pupil narrows. The muscles are antagonistic because they have opposite effects and work against each other: when one set contracts, the other relaxes.

A full answer includes

I wrote that the circular muscles contract.
I wrote that the radial muscles relax.
I wrote that the pupil narrows.
I wrote that antagonistic muscles have opposite effects: when one set contracts, the other relaxes.
I did not write that both sets contract together.

Also accept

circular contract, radial relax, pupil narrows; antagonistic: opposite effects, one contracts as the other relaxes

Watch for

“The radial muscles contract in bright light.” — Contracting radial muscles would widen the pupil. In bright light the circular muscles contract.

7. FK1 · Final check

Model answer. (a) S: the small pit in the retina, straight behind the middle of the lens, where the image of what you look at falls. (b) The fovea gives the most detailed vision; it is packed with cones. (c) T: the blind spot, where the optic nerve leaves the eye, has neither rods nor cones. U is ordinary retina, with many rods, and V is the lens.

A full answer includes

(a) I wrote S.
(b) I wrote that it gives the most detailed vision.
(c) I wrote T.
(c) I did not choose S, which is packed with cones.

Also accept

(a) S; (b) most detailed vision; (c) T

Watch for

“(c) S.” — S is the pit straight behind the lens: the fovea, packed with cones. The place with no receptors is where the cord leaves.
“(a) U.” — U is on the retina part way round from the pit, where there are many rods. The fovea is the pit straight behind the middle of the lens.

8. FK2 · Final check

Model answer. Distant objects are still clear, but near objects are blurred. The ciliary muscles cannot contract, so they stay relaxed and the suspensory ligaments stay tight. The lens stays thin, which is right for the nearly parallel rays from a distant object. A near object's rays are spreading and need more bending, so the thin lens cannot make them meet on the retina.

A full answer includes

I predicted that distant objects are clear.
I predicted that near objects are blurred.
I wrote that the suspensory ligaments stay tight.
I wrote that the lens stays thin.
I wrote that a thin lens cannot refract the spreading rays from a near object enough.
I did not predict that distant objects are blurred.

Also accept

distant clear, near blurred: ligaments tight, lens stays thin, too little refraction for near rays

Watch for

“Distant objects are blurred.” — Relaxed ciliary muscles and a thin lens are exactly what a distant object needs. It is near objects that need the muscles to contract.

9. FK3 · Final check

Model answer. Colour vision comes from the cones. There are normally three kinds, each absorbing light of a different colour, and the brain compares how strongly each kind responds. With one kind missing, red and green light give the same pattern of responses, so the two tomatoes look the same colour. The other cones still respond in bright light, so the shapes are seen clearly.

A full answer includes

I wrote that colour vision comes from the cones.
I wrote that there are normally three kinds of cone.
I wrote that each kind absorbs light of a different colour.
I wrote that with one kind missing, red and green light are not told apart.
I did not write that the rods are the cause in bright light.

Also accept

colour from three kinds of cone, each absorbing a different colour; one kind missing, red and green alike

Watch for

“The rods cannot see colour.” — That is true, but in bright daylight the cones do the seeing. The cause here is a missing kind of cone.

10. FK4 · Final check

Model answer. The cones are packed most tightly at the fovea and are few elsewhere. There are no rods at the fovea; they are most numerous a little way out from it and become fewer towards the edges of the retina. At the blind spot there are neither rods nor cones.

A full answer includes

I wrote that the cones are packed most tightly at the fovea.
I wrote that there are few cones elsewhere.
I wrote that there are no rods at the fovea.
I wrote that the rods are most numerous a little way out from the fovea.
I wrote that the rods become fewer towards the edges.
I wrote that the blind spot has no rods or cones.
I did not write that rods and cones are spread evenly.

Also accept

cones at the fovea; rods away from it; none at the blind spot

Watch for

“Rods and cones are spread evenly.” — They are not: the cones are packed at the fovea and the rods are more numerous away from it.