Answers · The eye

Focusing and the retina — answer key

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

1. FQ1 · Practice

Options as numbered on the pupil sheet: 1 — increases; 2 — slacken; 3 — relax; 4 — decreases; 5 — becomes thinner; 6 — contract; 7 — becomes fatter; 8 — are pulled tight.

Model answer. The watch is near, so its rays need more bending. The ciliary muscles contract, the suspensory ligaments slacken, the elastic lens becomes fatter and it refracts the light more.

A full answer includes

I matched the ciliary muscles to contract.
I matched the suspensory ligaments to slacken.
I matched the lens to becomes fatter.
I matched the refraction of light to increases.
I did not match the ciliary muscles to relax.

Also accept

ciliary contract; ligaments slacken; lens fatter; more refraction

Watch for

“The suspensory ligaments are pulled tight.” — The ligaments are pulled tight for a distant object. For a near object they slacken.
“The ciliary muscles relax.” — The ciliary muscles relax for a distant object. For a near object they contract.

2. FQ2 · Practice

Options as numbered on the pupil sheet: 1 — The lens refracts the light more.; 2 — The rays from the ball meet on the retina.; 3 — The suspensory ligaments slacken.; 4 — The lens springs into a fatter shape.; 5 — The ciliary muscles contract..

Model answer. The ball comes close, so its rays are spreading and need more bending. The ciliary muscles contract, so the suspensory ligaments slacken. With no pull on it, the elastic lens springs fatter, and the fatter lens refracts the light more, so the rays from the ball meet on the retina. Changing the shape of the lens in this way is accommodation.

A full answer includes

I put the ciliary muscles contracting first.
I put the ligaments slackening before the lens changes shape.
I put the lens refracting more before the rays meet on the retina.
I did not put the lens changing shape before the muscles act.

Also accept

muscles contract, ligaments slacken, lens fatter, more refraction, rays on retina

Watch for

“The lens gets fatter first, then the muscles contract.” — The muscles act first: their contracting lets the ligaments slacken, and that lets the lens spring fatter.

3. FQ3 · Practice

Model answer. B. At X the cones line has its tall, narrow peak and the rods line is at zero: that is the fovea, packed with cones. At W both lines fall to zero: that is the blind spot, with no receptors. At Y the rods line is at its highest. At Z, the edge, both lines are low.

A full answer includes

I chose row B.
I did not choose row A: the fovea is where the cones peak, not the rods.

Also accept

Watch for

“Row A: the fovea is at Y.” — At Y the rods line is high and the cones line is low. The fovea has no rods and is packed with cones: that is X.
“Row C: the blind spot is at Z.” — At Z there are still some rods and cones. The blind spot has none at all: both lines fall to zero at W.
“Row D: the fovea is at W.” — At W both lines fall to zero: no receptors, so no vision there. The fovea is packed with cones, at the cones' peak.

4. FQ4 · Practice

Model answer. Cones give colour vision. There are three different kinds, each absorbing light of a different colour. Rods do not give colour vision.

A full answer includes

I chose the cones.
I did not choose the rods, which are more sensitive but give no colour.

Also accept

cones

Watch for

“The rods.” — Rods are more sensitive to light, but they give no colour vision.
“Rods and cones.” — Only one of the two kinds of receptor gives colour; the other gives night vision.
“Optic nerve cells.” — Nerve cells carry impulses; they are not receptors and do not absorb light.

5. FQ5 · Practice

Model answer. The fovea gives the most detailed vision. It lies where the image of what you look at falls, and it is packed with cones.

A full answer includes

I chose: it gives the most detailed vision.
I did not choose the place where the optic nerve leaves, which is the blind spot.

Also accept

the most detailed vision

Watch for

“It is where the optic nerve leaves the eye.” — That is the blind spot, which has no receptors and sees nothing.
“It changes the shape of the lens.” — The ciliary muscles and suspensory ligaments change the lens's shape. The fovea is part of the retina.
“It controls how much light enters the eye.” — The iris does that, by changing the size of the pupil. The fovea is part of the retina.

6. FQ6 · Practice

Model answer. Rods are more sensitive to light than cones, so they give night vision. They give no colour vision, there is only one kind, and there are none in the fovea.

A full answer includes

I chose: they are more sensitive to light than cones, and give night vision.
I did not choose the statement about three kinds: that is true of cones.

Also accept

more sensitive than cones; night vision

Watch for

“They are less sensitive to light than cones, and give colour vision.” — Both halves describe cones. Rods are the more sensitive receptors and give no colour.
“They are packed most tightly in the fovea, and give detailed vision.” — The fovea is packed with cones and has no rods.
“They come in three kinds, each absorbing light of a different colour.” — That describes the cones, which give colour vision. There is only one kind of rod.

7. FQ7 · Practice

Model answer. (a) The lens is still fat from focusing on the near map, and a fat lens refracts light a lot. The rays from the mountain are nearly parallel and need little bending, so they are bent too much and meet in front of the retina. (b) The lens must become thinner, so that it refracts the light less and the rays meet on the retina.

A full answer includes

(a) I predicted that the rays meet in front of the retina.
(a) I wrote that the fat lens refracts the nearly parallel rays too much.
(b) I wrote that the lens must become thinner.
(b) I wrote that a thinner lens refracts light less, so the rays meet on the retina.
(a) I did not predict that the rays meet behind the retina.

Also accept

(a) in front of the retina; (b) thinner, refracting less

Watch for

“(a) Behind the retina.” — Rays meet behind the retina when they are bent too little. A fat lens bends parallel rays too much, so they meet in front of it.

8. FQ8 · Practice

Model answer. For a distant object the ciliary muscles relax, they do not contract. The relaxed ring of muscle is wider, so the suspensory ligaments are pulled tight. They pull on the edge of the lens and make it thinner, so it refracts the light less.

A full answer includes

I wrote that for a distant object the ciliary muscles relax.
I wrote that the ligaments are pulled tight when the ring of muscle relaxes and widens.
I wrote that the tight ligaments pull the lens thinner.
I did not write that contracting muscles pull the ligaments tight.

Also accept

muscles relax, not contract; ligaments then pulled tight; lens thinner

Watch for

“The ligaments slacken for a distant object.” — The ligaments are pulled tight for a distant object; they slacken for a near one.

9. FQ9 · Practice

Model answer. Rods are most numerous a little way out from the fovea, where there are few cones. The fovea itself is packed with cones and has no rods, the blind spot has neither, and the rods become fewer towards the edge of the retina.

A full answer includes

I chose: a little way out from the fovea.
I did not choose the middle of the fovea, which has no rods.

Also accept

a little way out from the fovea

Watch for

“In the middle of the fovea.” — The fovea has no rods: it is packed with cones.
“Exactly at the blind spot.” — The blind spot has no receptors at all.
“At the very edge of the retina.” — There are rods near the edge, but fewer of them: they are most numerous closer in.

10. FQ10 · Practice

Model answer. The cones, which give colour vision, need brighter light and hardly respond at dusk. You see mainly with the rods, which are more sensitive to light, so you can still see the flower's shape; but rods give no colour vision.

A full answer includes

I wrote that in dim light the cones hardly respond.
I wrote that you see mainly with the rods, which are more sensitive to light.
I wrote that rods give no colour vision.
I wrote that colour vision needs the cones.
I did not write that the rods stop working at dusk.

Also accept

cones need bright light; rods sensitive but no colour

Watch for

“The rods stop working at dusk.” — Rods are the more sensitive receptors: they keep working in dim light. It is the cones that stop responding.

11. FQ11 · Practice

Options as numbered on the pupil sheet: 1 — blind spot; 2 — cornea; 3 — fovea; 4 — optic nerve; 5 — lens; 6 — retina.

Model answer. J is the fovea, the small pit on the line of sight at the back of the eye, packed with cones. K is the blind spot, where the optic nerve leaves and there are no receptors. L is the retina, the light-sensitive lining, near the front edge of the back of the eye. M is the lens, the clear oval behind the pupil.

A full answer includes

I matched J to the fovea.
I matched K to the blind spot.
I matched L to the retina.
I matched M to the lens.
I did not swap J and K: one is on the line of sight, the other is where the cord leaves.

Also accept

J fovea; K blind spot; L retina; M lens

Watch for

“J is the blind spot.” — J is on the line of sight, where the image of what you look at falls: the fovea. The blind spot is where the cord leaves, at K.
“K is the optic nerve.” — The optic nerve is the thick cord outside the eye. K is on the lining itself, where the nerve leaves: the blind spot, with no receptors.
“L is the cornea.” — The cornea is the clear window at the very front. L is on the lining at the back of the eye: the retina.

12. FT1 · Apply

Model answer. Looking straight at the star puts its image on the fovea, which has only cones; cones need brighter light. Looking slightly to one side puts the image a little way out from the fovea, where rods are most numerous. Rods are more sensitive, so they respond to the faint light.

A full answer includes

I wrote that looking straight at it puts the image on the fovea, which has only cones.
I wrote that looking to one side puts the image a little way out from the fovea.
I wrote that there are many rods there.
I wrote that rods are more sensitive to dim light than cones.
I did not write that the image then falls on the blind spot.

Also accept

image away from fovea; many rods there; rods more sensitive

Watch for

“The image falls on the blind spot when she looks straight at it.” — Looking straight at something puts its image on the fovea, not the blind spot. The fovea has cones and no rods.

13. FT2 · Apply

Model answer. The eye's lens stays in place and changes shape instead. For a near object the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fatter, refracting light more; for a distant object the muscles relax, the ligaments are pulled tight and the lens becomes thinner, refracting light less.

A full answer includes

I wrote that the eye changes the shape of its lens, not its position.
Near: I wrote that the ciliary muscles contract.
Near: I wrote that the suspensory ligaments slacken.
Near: I wrote that the lens becomes fatter.
Near: I wrote that the lens refracts light more.
Distant: I wrote that the ciliary muscles relax.
Distant: I wrote that the suspensory ligaments are pulled tight.
Distant: I wrote that the lens becomes thinner.
Distant: I wrote that the lens refracts light less.
I did not write that the eye moves its lens.

Also accept

eye changes lens shape; near fatter, more refraction; far thinner, less

Watch for

“The eye moves its lens forwards for near objects.” — The eye's lens does not move. Its shape changes.

14. FT3 · Apply

Model answer. (a) Rods are more sensitive to light than cones, so with many rods the owl sees well in dim light, when it hunts. (b) With few cones, the owl has poor colour vision.

A full answer includes

(a) I wrote that rods are more sensitive to light.
(a) I wrote that the owl sees well in dim light, when it hunts.
(b) I wrote that with few cones the owl has poor colour vision.
(b) I did not write that the owl cannot see at night.

Also accept

(a) sees well in dim light; (b) poor colour vision

Watch for

“(b) It cannot see at night.” — Many rods help it see at night. The cost is colour vision, which needs cones.

15. FT4 · Apply

Model answer. The fovea is the part of the retina that gives the most detailed vision, where the image of what the eagle looks at falls. With even more cones packed into it, the image there is even more detailed, so the eagle can make out a small mouse a long way below.

A full answer includes

I wrote that the fovea gives the most detailed vision.
I wrote that more cones packed in it give even more detail.
I wrote that the eagle can then make out small, distant things like the mouse.
I did not write that more cones let it see in the dark.

Also accept

fovea gives detail; more cones, more detail; sees small far objects

Watch for

“More cones let it see in the dark.” — Cones need bright light. More cones in the fovea give more detail.