Sense organ — receptor cells for one stimulus
Sense organ: a group of receptor cells that responds to one specific stimulus. The five in 0610 scope are light (eye), sound (ear), touch and pressure (skin), temperature change (skin) and chemicals (tongue for taste, nose for smell). The eye is the light detector — rods and cones in the retina convert light energy into electrical impulses, which the optic nerve carries to the brain. Core parts you must label: cornea, iris, pupil, lens, retina, optic nerve, blind spot.
Cornea and lens refract; iris and pupil regulate
Cornea and lens are the refracting pair: the transparent cornea bends light entering the eye and supplies most of the focusing power; the biconvex lens, slung from the suspensory ligaments, fine-focuses onto the retina. The iris is a ring of muscle setting pupil diameter; the pupil is only the hole light passes through. The retina is the light-sensitive layer of rods and cones; the optic nerve carries impulses to the brain; the blind spot is its exit point, with no receptors.
Pupil reflex — two antagonistic iris muscles
Pupil reflex: an involuntary, protective response driven by two antagonistic muscle sets in the iris. In bright light the circular muscles contract and the radial muscles relax, so the pupil constricts — less light enters and the retina is protected. In dim light the radial muscles contract and the circular muscles relax, so the pupil dilates and more light enters, letting the retina still form an image. Like every reflex arc it is rapid, automatic and protective.
Drawn from real examiner reports.
Cornea dropped from refraction
Asked what refracts light in the eye, candidates write lens only, or name the iris. Both the cornea and the lens bend light onto the retina, and the cornea supplies most of the refractive power because light bends most sharply passing from air into the denser cornea. The lens then makes the fine adjustment. Always write cornea and lens.
Cornea omitted when refraction was asked for; many wrote "lens only" or "iris". (w22 Papers 12/22 Q26 — cambridge-0610.md Section 14)
Pupil ≠ a focusing structure
The pupil is a hole in the centre of the iris, not tissue, so it cannot bend light. Writing that the pupil focuses light scores nothing. The pupil governs how much light enters; the cornea and lens govern where that light is focused. The iris earns marks for controlling pupil diameter, never for focusing.
Candidates stated "the pupil focuses light" when the pupil only controls light entry. (s22 Paper 13 Q28; s23 Paper 32 Q6b i — cambridge-0610.md Section 14)
Blind spot described, not located
"The part of the eye that cannot see" describes the effect and earns nothing. Full credit needs the location plus the reason: the blind spot is the point on the retina where the optic nerve leaves the eye, and there are no rods or cones there, so light falling on it cannot be detected and no impulse is sent to the brain.
Few candidates knew the blind spot is where the optic nerve leaves the retina (no rods or cones there). (s23 Paper 32 Q6b ii; s22 Paper 42 Q3b ii — cambridge-0610.md Section 14)
Rods and cones swapped (Extended)
The recurring reversal is "rods detect colour, cones detect dim light". It is the other way round: rod cells respond to low light intensity and give black-and-white vision only; cone cells need bright light and give colour vision, in three types for red, green and blue. Learn one half securely and derive the other.
Rods vs cones functions swapped by many candidates; cone cells found in the fovea. (s23 Paper 23 Q25; s22 Paper 42 Q3b i — cambridge-0610.md Section 14)
Blind spot ≠ fovea (Extended)
These are opposite extremes on the same retina, not the same place. The blind spot has no receptors at all, because it is where the optic nerve exits. The fovea is a small central region with the highest density of cone cells and no rods, giving the sharpest colour vision. Rods dominate the periphery; cones dominate the fovea.
Constrict and dilate written back to front
Constrict means the pupil gets smaller, in bright light. Dilate means it gets larger, in dim light. A frequent multiple-choice trap pairs the correct muscle actions with the wrong outcome — for example circular muscles contract and the pupil dilates. Contracting circular muscles always close the pupil down.
Name the structure, not the colour
Do not write "the black bit" or "the coloured bit". Use the labelled term: cornea, iris, pupil, lens, retina, optic nerve, blind spot. Marks are awarded for the named structure, never for a description of how it looks.
One function, in mark-scheme words
Cornea refracts light. Iris controls pupil diameter. Pupil is the hole light passes through. Lens fine-focuses onto the retina. Retina holds rods and cones that convert light to electrical impulses. Optic nerve carries impulses to the brain. Blind spot has no receptors.
Reflex answers need both muscle sets
Bright light: circular muscles contract, radial muscles relax, pupil constricts. Dim light: radial muscles contract, circular muscles relax, pupil dilates. Naming only one muscle set, or giving only the resulting pupil size, typically loses marks.
Accommodation: keep the chain in order
(Extended) Write muscle, then ligament, then lens shape, then refractive power, then focus. Near object: ciliary muscle contracts, suspensory ligaments slacken, lens thickens, refraction increases. Distant object: the reverse. "The lens changes shape" alone is not enough.
Sense organ: a group of receptor cells that responds to a specific stimulus.
| Stimulus | Sense organ |
|---|---|
| Light | Eye |
| Sound | Ear |
| Touch | Skin |
| Temperature (change) | Skin |
| Chemicals | Tongue (taste) / nose (smell) |
The eye is the sense organ for light. Light-sensitive receptor cells (rods and cones) in the retina convert light energy into electrical impulses, carried to the brain by the optic nerve.
(Extended) Compare rod cells and cone cells: sensitivity, type of vision, and distribution in the retina.
(a) Define the term sense organ.
(b) Identify the stimulus detected by the eye, and name the structure that converts this stimulus into electrical impulses.
