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- The eye: focusing and the pupil reflex
The eye: focusing and the pupil reflex
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The eye is a sense organ containing light receptors. It changes shape to focus and changes pupil size to control light.
Structure of the eye
Iris: coloured muscle that controls the size of the pupil.
Lens: focuses light onto the retina. It is held by suspensory ligaments attached to ciliary muscles.
Retina: contains receptor cells: rods (work in dim light, black and white) and cones (colour, in bright light).
Optic nerve: carries impulses to the brain.
Focusing (accommodation)
Responding to light intensity
Dim light: the radial muscles contract and the circular muscles relax, so the pupil gets wider, letting in more light.
This is a reflex: it is fast and automatic.
Describe what happens in the eye when you look up from a book to a distant hill.
- The ciliary muscles relax.
- The suspensory ligaments are pulled tight.
- The lens is pulled thinner, so it refracts light less.
Answer: Ciliary muscles relax, ligaments tighten, lens becomes thinner.
Cornea and lens refract light onto the retina, where rods and cones are receptors. Near: ciliary muscles contract, ligaments slacken, lens fatter. Distant: muscles relax, ligaments tighten, lens thinner. Bright light: circular muscles contract and the pupil narrows.
The interactive lesson includes the diagrams for this topic.
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