IGCSE · Practice questions · Sense organs and the eye

Bend, focus, detect.

Six original Cambridge-style questions on receptors, the structure and functions of the eye, the pupil reflex, accommodation and the distribution of rods and cones.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two reflexes, two muscle sets.

Cornea refracts; lens focuses on to the retina; iris controls light entering the pupil.
Bright light: pupil smaller; (Extended) circular muscles contract, radial relax.
Near object (Extended): ciliary muscles contract, ligaments slack, lens fat, more refraction.
Rods (Extended): dim light, not at the fovea. Cones: three kinds, colour, packed in the fovea.

Recall

[3 marks]

Describe what is meant by a sense organ and name the sense organ that responds to each of: (a) light, (b) chemicals in food.

A sense organ is a group of receptor cells that responds to a specific stimulus. ✓
(a) The eye. ✓
(b) The tongue (also accept the nose for chemicals in the air). ✓

Recall

[4 marks]

State the function of each of these parts of the eye: cornea, iris, lens, optic nerve.

Cornea: refracts light (as it enters the eye). ✓
Iris: controls how much light enters the pupil. ✓
Lens: focuses light on to the retina. ✓
Optic nerve: carries impulses from the retina to the brain. ✓

Explain

[4 marks]

A bright torch is shone into a person's eye. Describe and explain the change in the pupil. (Extended candidates should refer to the iris muscles.)

The pupil becomes smaller (its diameter decreases). ✓
This reduces the amount of light entering the eye, which protects the retina from damage. ✓
It is a reflex: automatic and rapid, triggered by light receptors in the retina. ✓
Extended: the circular muscles of the iris contract and the radial muscles relax; they act antagonistically. ✓

Explain

[5 marks]

Extended. Explain how the eye changes so that a person can focus on a page held 25 cm from the eye after looking at a distant mountain.

Light from the near page is diverging, so it needs more refraction to focus on the retina. ✓
The ciliary muscles contract. ✓
This reduces the tension in the suspensory ligaments, which go slack. ✓
The elastic lens becomes fatter (more convex). ✓
The fatter lens refracts light more, so the image is focused on the retina. ✓

Data

[4 marks]

Extended. The table shows the number of receptors in small areas of the human retina at different distances from the fovea.

Distance from fovea / degrees	Cones / thousands per mm²	Rods / thousands per mm²
0	150	0
20	8	150
60	5	80

(a) Describe the distribution of rods and cones. (b) Use the data to explain why a dim star is easier to see when you look slightly to one side of it.

(a) Cones are concentrated at the fovea and are few elsewhere. ✓
Rods are absent from the fovea, are most numerous a short distance from it and become fewer towards the edge of the retina. ✓
(b) Rods are more sensitive to dim light than cones (they give night vision). ✓
Looking slightly to one side puts the star's image on a region with many rods, instead of the fovea, which has no rods. ✓

Analysis

[3 marks]

Extended. Explain why we can see colour clearly in daylight but not in very dim light.

Colour vision needs cones; there are three kinds, each absorbing light of a different colour. ✓
Cones only work in bright light. ✓
In dim light only the rods respond, and they do not distinguish colours, so we see in shades of grey. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.