The pupil reflex — questions

1. describe

You are reading outdoors on a cloudy day when a cloud moves away from the sun, and the light at your eyes becomes much brighter. Which describes what your pupils do?

Circle one answer.

• They stay the same: only the lens changes.
• They get narrower: their diameter decreases.
• They get wider: their diameter increases.
• The whole eyeball gets smaller instead.

Nudge (only if you are stuck): Does your eye need more light, or less, when the light becomes brighter?

2. explain

Why does the pupil narrow when the light becomes bright?

Circle one answer.

• Less light enters, so the retina is protected from damage.
• Less light enters, so the lens is protected from damage.
• The pupil shuts, so no light at all can reach the retina.
• More light enters, so the retina gets enough light to see.

Nudge (only if you are stuck): A narrower hole lets in less light. What could too much light harm?

3. determine

A magnified front view of an eye has a millimetre scale beside it. The left edge of the pupil is at the 2.5 mm mark and the right edge is at the 7.8 mm mark. Determine the pupil diameter.

Pupil diameter: ______________ unit ________

Nudge (only if you are stuck): The diameter is the distance from one edge of the pupil to the other.

4. describe

A student's pupil was measured at five light intensities. Which describes the pattern in the table?

Circle one answer.

• Diameter falls as the light gets brighter; the biggest drop is at the dim end.
• Diameter rises as the light gets brighter; the biggest rise is at the dim end.
• Diameter falls as the light gets brighter; the biggest drop is at the bright end.
• Diameter falls as the light gets brighter, by the same amount each time.

Nudge (only if you are stuck): Read down the diameters, then find the size of each drop from one row to the next.

5. identify

Match each word to the right part of the pupil reflex.

Write the number of the matching option.

1. the muscles of the iris
2. a change in pupil diameter
3. a change in light intensity
4. light receptors in the retina

• stimulus: ______
• receptors: ______
• effector: ______
• response: ______

Nudge (only if you are stuck): Which part is the change, which detects it, which acts, and what is the result?

6. identify

Supplement: which row shows what the two sets of iris muscles do in bright light and in dim light?

Circle one answer.

• A
• B
• C
• D

Nudge (only if you are stuck): In bright light the pupil narrows. Which set of muscles forms rings round it?

7. explain

A student says: "When I walked into the sunshine, I made my pupils smaller on purpose." Explain why this is wrong.

Nudge (only if you are stuck): Can you choose the size of your pupils?

8. identify / calculate

A student measured the diameter of one pupil four times in a dim room: 6.2 mm, 6.6 mm, 3.7 mm and 6.3 mm. (a) Identify the anomalous reading. (b) Calculate the mean diameter, leaving out the anomalous reading. Give your answer to 1 decimal place.

(a) Anomalous reading: ______________ unit ________

(b) Mean diameter: ______________ unit ________

Nudge (only if you are stuck): Which reading does not fit the others? Leave it out, then add the rest and divide by how many there are.

9. explain

Supplement: explain why the iris needs two sets of muscles, circular and radial, to change the size of the pupil.

Nudge (only if you are stuck): What can a muscle do when it contracts, and what can it not do?

10. calculate

Use the table to calculate the percentage decrease in pupil diameter from 1 lux to 10 000 lux. Give your answer to 2 significant figures.

Percentage decrease: ______________ unit ________

Nudge (only if you are stuck): Find the decrease first, then divide it by the diameter you started with.

11. calculate

A swimmer's pupil is 2.4 mm across in bright sunshine and 7.2 mm across in a dark changing room. The area of a circular pupil is π × (diameter ÷ 2)². Calculate how many times larger the pupil's area is in the changing room than in the sunshine.

Times larger: ______________ unit ________

Nudge (only if you are stuck): Work out both areas, then divide the larger by the smaller.

12. state / predict

Supplement: a hiker walks from a sunny field into a shady forest. (a) State what the circular muscles of her iris do. (b) State what the radial muscles do. (c) Predict what happens to her pupil diameter.

(a) Circular muscles

(b) Radial muscles

(c) Pupil diameter

Nudge (only if you are stuck): Is the light at her eyes getting brighter or dimmer? Which way must her pupils change?

13. predict / explain

Supplement: eye drops can relax the circular muscles of the iris, so that these muscles cannot contract. Predict what happens to the pupil when the drops are put into an eye in a well-lit room, and explain.

Nudge (only if you are stuck): Which set of muscles is still free to contract, and which way does it pull?