How does the eye detect light, and how does it focus on near and distant objects?
The structure of the eye and the function of its parts, how the eye focuses light (accommodation), and the pupil reflex.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on the eye, covering the structure and functions of the parts of the eye, how the eye focuses light by accommodation, and the pupil reflex that controls light entering the eye.
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What this dot point is asking
WJEC Double Award Unit 4 wants you to describe the structure of the eye and the function of its parts, explain how the eye focuses (accommodation), and describe the pupil reflex.
The structure of the eye
The cornea does most of the focusing, and the lens makes the fine adjustments.
How the eye focuses (accommodation)
- Near object: the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter and more curved, refracting the light more.
- Distant object: the ciliary muscles relax, the suspensory ligaments tighten, and the lens becomes thinner and less curved, refracting the light less.
The pupil reflex
The pupil reflex is an automatic response that controls how much light enters the eye, protecting the retina:
- Bright light: the circular muscles of the iris contract, making the pupil smaller, so less light enters and the retina is protected.
- Dim light: the radial muscles contract, making the pupil larger, so more light enters and you can see better.
Because it is automatic and protective, the pupil reflex is a good example of a reflex action.
How an image forms on the retina
Light from an object is refracted (bent) first by the cornea and then fine-tuned by the lens, so that the rays meet to form a sharp image on the retina. The image formed is actually upside down, and the brain interprets it the right way up. The receptor cells in the retina detect the light and send impulses along the optic nerve to the brain, which builds the picture we see. Understanding that focusing means bringing the light rays together on the retina explains why the lens must change shape for objects at different distances.
Short and long sight
Sometimes the eye cannot focus light exactly on the retina. In short sight, distant objects look blurred because the light is focused in front of the retina; it is corrected with a concave (diverging) lens. In long sight, near objects look blurred because the light would focus behind the retina; it is corrected with a convex (converging) lens. Glasses or contact lenses add the right amount of refraction so the image lands on the retina. Linking the position where the light focuses to the type of correcting lens is a common application question.
Try this
Q1. Name the part of the eye that carries impulses to the brain. [1 mark]
- Cue. The optic nerve.
Q2. What happens to the lens when focusing on a near object? [1 mark]
- Cue. It becomes fatter (more curved).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how the eye focuses on a near object compared with a distant object.Show worked answer →
A Unit 4 describe question worth 4 marks. Reward: to focus on a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter (more curved) to refract the light more (2); to focus on a distant object, the ciliary muscles relax, the suspensory ligaments tighten, and the lens becomes thinner (less curved) to refract the light less (2). Markers credit the ciliary muscle, ligaments and lens shape for each. A common error is to get the muscle action the wrong way round.
WJEC style3 marksExplain how the eye responds to bright light using the pupil reflex.Show worked answer →
A Unit 4 explain question. Reward: in bright light, the circular muscles of the iris contract and the radial muscles relax (1); this makes the pupil smaller (1); so less light enters the eye, protecting the retina from damage (1). Markers credit the circular muscles contracting, the smaller pupil and less light entering. A common error is to confuse the circular and radial muscles or to say the pupil gets bigger in bright light.
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