Answers · The eye

Your eye and its parts — answer key

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

1. EQ1 · Practice

Options as numbered on the pupil sheet: 1 — chemicals; 2 — light; 3 — sound; 4 — touch and temperature.

Model answer. The eye responds to light, the ear to sound and the skin to touch and temperature. The tongue and the nose both respond to chemicals: the tongue to chemicals in food and drink, the nose to chemicals in the air.

A full answer includes

I matched the eye to light.
I matched the ear to sound.
I matched the skin to touch and temperature.
I matched both the tongue and the nose to chemicals.
I did not leave the tongue or the nose unmatched because chemicals was already used once.

Also accept

eye light; ear sound; skin touch and temperature; tongue chemicals; nose chemicals

Watch for

“The nose responds to air.” — The nose responds to chemicals carried in the air, which is how you smell things.
“Each stimulus goes with only one sense organ.” — Two organs respond to chemicals: the tongue to chemicals in food and drink, the nose to chemicals in the air.

2. EQ2 · Practice

Model answer. A sense organ is a group of receptor cells that respond to a specific stimulus: the eye's receptors respond to light, the ear's to sound.

A full answer includes

I chose: a group of receptor cells that respond to one kind of stimulus.
I did not choose the single receptor cell.

Also accept

a group of receptor cells responding to one kind of stimulus

Watch for

“A single receptor cell that responds to one kind of stimulus.” — A sense organ holds many receptor cells, grouped together. The retina of one eye has millions.
“A group of receptor cells that respond to every kind of stimulus.” — Each sense organ responds to one kind of stimulus: the ear's receptors respond to sound but not to light.
“A group of nerve cells that carry the impulses to the brain.” — That describes a nerve. A sense organ is made of receptor cells, which detect a stimulus and start the impulses.

3. EQ3 · Practice

Model answer. The iris is the coloured ring of muscle, and its colour differs from person to person. The pupil is a hole and looks black in everyone; the cornea and the lens are clear.

A full answer includes

I chose the iris.
I did not choose the pupil, which is a hole and looks black.

Also accept

iris

Watch for

“The pupil.” — The pupil is a hole, so it looks black in everyone's eye. The coloured ring around it is the iris.
“The cornea.” — The cornea is a clear window, so you see the colour behind it, not a colour of its own.
“The lens.” — The lens is clear and lies behind the pupil, so its colour does not show from the front.

4. EQ4 · Practice

Options as numbered on the pupil sheet: 1 — controls how much light enters the pupil; 2 — refracts light as it enters the eye; 3 — carries impulses to the brain; 4 — focuses light on to the retina; 5 — contains light receptors, some sensitive to different colours.

Model answer. The cornea refracts (bends) the light as it enters. The iris controls how much light enters through the pupil. The lens focuses the light on to the retina. The retina contains the light receptors, some sensitive to light of different colours. The optic nerve carries impulses to the brain.

A full answer includes

I matched the cornea to refracting light.
I matched the iris to controlling how much light enters the pupil.
I matched the lens to focusing light on to the retina.
I matched the retina to containing light receptors.
I matched the optic nerve to carrying impulses to the brain.
I did not match the cornea to focusing light on to the retina.

Also accept

cornea refracts; iris controls light entering; lens focuses on retina; retina has receptors; optic nerve carries impulses

Watch for

“The optic nerve carries light to the brain.” — No light reaches the brain. The optic nerve carries impulses, started by the receptors in the retina.
“The cornea focuses light on to the retina.” — Focusing on to the retina is the lens's job. The cornea refracts the light first, as it enters.

5. EQ5 · Practice

Model answer. The optic nerve. The retina is healthy, so its receptors start impulses when light falls on them. The optic nerve carries impulses to the brain, so if none arrive, the nerve is the part that is not working.

A full answer includes

I chose the optic nerve.
I did not choose the blind spot: every healthy eye has one.

Also accept

optic nerve

Watch for

“The iris.” — The iris controls how much light enters. The light already reaches the retina, so the iris is doing its job.
“The blind spot.” — Every healthy eye has a blind spot, and the rest of the retina still starts impulses. Here the impulses fail on their way to the brain.
“The pupil.” — The pupil is a hole, and the light passes through it normally.

6. EQ6 · Practice

Model answer. The pupil is a hole in the middle of the iris. Light enters the eye through it, and it looks black because very little light comes back out.

A full answer includes

I chose: it is a hole in the iris that lets light into the eye.
I did not choose the coloured ring, which is the iris.

Also accept

a hole in the iris that lets light in

Watch for

“It is the coloured ring of muscle in front of the lens.” — The coloured ring is the iris. The pupil is the hole in its middle.
“It is the clear layer that bends light as it enters.” — That is the cornea, the clear window at the very front of the eye.
“It is a black layer at the back that detects light.” — The layer at the back that detects light is the retina. The pupil looks black because it is a hole: light goes in and very little comes back out.

7. EQ7 · Practice

Model answer. The image is formed on the retina, at the back of the eye. The lens does not hold the image: it focuses the light, bending the rays so that they meet on the retina.

A full answer includes

I wrote that the image is formed on the retina, not on the lens.
I wrote that the lens focuses the light on to the retina.
I did not write that the image is formed in the brain.

Also accept

image on the retina; the lens focuses light on to the retina

Watch for

“The image is formed in the brain.” — The image is formed on the retina. The brain receives impulses from it along the optic nerve.

8. EQ8 · Practice

Model answer. (a) The blind spot: the place where the optic nerve leaves the eye. (b) There is no room for light receptors where the nerve fibres gather, so the blind spot has none. Light falling on it starts no impulses, so no impulses about that part of the scene reach the brain.

A full answer includes

(a) I wrote the blind spot.
(b) I wrote that the blind spot has no light receptors.
(b) I wrote that light falling there starts no impulses.
(b) I did not write that the optic nerve itself detects the light.

Also accept

(a) blind spot; (b) no light receptors, so no impulses

Watch for

“(a) The optic nerve.” — The patch is the place on the retina where the nerve leaves: the blind spot. The optic nerve is the bundle behind it, running to the brain.
“(b) The optic nerve detects the light there.” — Only receptors start impulses. The optic nerve carries impulses but cannot detect light.

9. EQ9 · Practice

Model answer. Each sense organ is a group of receptor cells that respond to a specific stimulus. The receptors in the ear respond to sound and do not respond to light; only the receptors in the eye respond to light.

A full answer includes

I wrote that the receptors in a sense organ respond to a specific stimulus.
I wrote that the ear's receptors respond to sound.
I wrote that the ear's receptors do not respond to light.
I did not write that light cannot reach the ear.

Also accept

each sense organ responds to a specific stimulus; ear sound, not light

Watch for

“The ear is too far from the light.” — Light reaches your ears too. Their receptors simply do not respond to light: they respond to sound.

10. EQ10 · Practice

Options as numbered on the pupil sheet: 1 — blind spot; 2 — lens; 3 — cornea; 4 — pupil; 5 — iris; 6 — retina; 7 — optic nerve.

Model answer. Start where the light enters, on the left. 3 is the cornea, the clear curved window at the very front. 1 is the iris, one half of the coloured ring, and 6 is the pupil, the gap between the two halves. 5 is the lens, behind the pupil. At the back, 2 is the optic nerve, the thick cord leaving the eye, and 4 is the blind spot, the place on the retina where the nerve leaves. The retina itself, the lining of the back of the eye, is not numbered.

A full answer includes

I matched 1 to the iris.
I matched 2 to the optic nerve.
I matched 3 to the cornea.
I matched 4 to the blind spot.
I matched 5 to the lens.
I matched 6 to the pupil.
I did not swap 2 and 4: one is the cord itself, the other the place on the lining where it leaves.

Also accept

1 iris; 2 optic nerve; 3 cornea; 4 blind spot; 5 lens; 6 pupil

Watch for

“4 is the optic nerve.” — Dot 4 is on the lining, where the cord joins it: that place is the blind spot. The optic nerve is the cord itself.
“6 is the iris.” — Dot 6 is in the gap between the two halves of the coloured ring: the gap is the pupil, the halves are the iris.

11. ET1 · Apply

Model answer. The receptors respond when the whisker touches something, so the stimulus is touch. In a human, the skin contains receptors that respond to touch.

A full answer includes

I wrote touch.
I wrote the skin.
I did not write sound or the ear.

Also accept

touch; skin

Watch for

“Sound, so the ear.” — Nothing in the whisker responds to sound: it responds when it is bent by touching something.

12. ET2 · Apply

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

Model answer. The ring that changes the hole does the iris's job, and the hole itself is like the pupil. The light sensor detects light, like the retina; its cells for red, green and blue light are like the retina's receptors, some sensitive to light of different colours. The cable carries signals to the computer, as the optic nerve carries impulses to the brain.

A full answer includes

I matched the ring to the iris.
I matched the hole to the pupil.
I matched the light sensor to the retina.
I matched the cable to the optic nerve.
I did not match the light sensor to the lens.

Also accept

ring iris; hole pupil; sensor retina; cable optic nerve

Watch for

“The light sensor is the lens.” — The lens focuses the light; the part that detects it is the retina.
“The ring is the pupil.” — The ring changes the size of the hole, as the iris changes the size of the pupil. The hole itself is like the pupil.

13. ET3 · Apply

Model answer. From the front you can see the iris, the coloured ring, and the pupil, the black hole in its middle. The lens is clear, and the retina and the optic nerve are at the back of the eye, out of sight.

A full answer includes

I chose the iris and the pupil.
I did not choose a pair with the retina, which lines the back of the eye.

Also accept

the iris and the pupil

Watch for

“The pupil and the retina.” — You look in through the pupil, but the retina lines the back of the eye, out of sight in an ordinary photograph.
“The iris and the lens.” — The lens is clear and lies behind the pupil, so it does not show: the black circle is the pupil.
“The retina and the optic nerve.” — Both are at the back of the eye, out of sight from the front.