Sense organs — Cambridge IGCSE Biology
Sense organs and the structure and function of the eye, the pupil reflex, accommodation, and rods, cones and the fovea.
Revision notes
4 short notes, in the order of the specification. Each one in short:
The eye is a sense organ: a group of receptor cells responding to light. The cornea refracts light as it enters, the iris controls how much light enters the pupil, and the lens focuses light on to the retina. The retina contains light receptors, and the optic nerve carries impulses to the brain. The blind spot has no receptors.
The pupil reflex is an automatic response that changes the diameter of the pupil when light intensity changes. In bright light the pupil constricts, letting in less light and protecting the retina. In dim light it dilates, letting in more light. The iris has antagonistic muscles: circular muscles contract to narrow the pupil and radial muscles contract to widen it.
Accommodation
SupplementAccommodation is changing the shape of the lens to focus on near or distant objects. For a near object, the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes thicker, so light is refracted more. For a distant object, the ciliary muscles relax, the ligaments are pulled tight and the lens becomes thinner, refracting light less.
Rods, cones and the fovea
SupplementRods and cones are the two types of light receptor in the retina. Rods are more sensitive to light, so they give night vision in dim light. Cones give colour vision in bright light, and there are three kinds, absorbing red, green and blue light. The fovea has the most cones and no rods, so it gives the sharpest vision.
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Quick check questions
Which structure of the eye refracts light as it enters?
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The cornea.
What is the blind spot?
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The place where the optic nerve leaves the eye. It has no receptors.
What happens to the pupil in bright light?
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It gets smaller, so less light enters the eye.
Supplement Which muscles of the iris contract to widen the pupil?
Show answer
The radial muscles.
Supplement What is the function of the fovea?
Show answer
It gives the sharpest vision because it has a high density of cones.
Frequently asked questions
How does the eye focus on near objects?
Supplement The eye focuses on near objects by accommodation. The ciliary muscles contract, so the tension in the suspensory ligaments decreases and they go slack. The lens becomes thicker and more curved, so it refracts light more and focuses the image on the retina. Remember: contracted muscle, slack ligaments, fat lens.
What does the cornea do?
The cornea refracts, or bends, light as it enters the eye. After the cornea, light passes through the pupil, and the lens focuses it on to the retina. The iris controls how much light enters. In exams use the exact verb: the cornea refracts light, while the lens focuses light.
Why does the pupil get smaller in bright light?
The pupil gets smaller in bright light to protect the retina from damage. This is the pupil reflex, an automatic response. Less light enters the eye when the pupil constricts. In the iris, the circular muscles contract and the radial muscles relax, which makes the pupil narrower. In dim light the reverse happens.
What is the difference between rods and cones?
Supplement Rods are more sensitive to light and are used for night vision in dim light, while cones give colour vision in bright light. There is one type of rod but three kinds of cone, absorbing red, green and blue light. Cones are packed closely in the fovea; rods are spread over the retina, with none in the fovea.
Why is there a blind spot in the eye?
The blind spot is the place where the optic nerve leaves the eye. It has no light receptors, so it is not sensitive to light, and any light falling on it is not detected. The optic nerve carries impulses, not images, from the receptors in the retina to the brain.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026