Learning outcomes
- Describe image formation in the normal eye.
- Draw simple ray diagrams for short and long sight.
- Explain correction using diverging and converging lenses.
- Relate focusing to lens shape and retina position.
12.1 Normal vision
Light enters through the cornea and pupil. The cornea provides much of the refraction, while the eye lens fine-tunes the focus. A real, inverted image forms on the retina, where light-sensitive cells convert it into electrical signals. The brain interprets these signals.
The iris controls pupil size and therefore the amount of light entering. Focusing on objects at different distances is called accommodation. Ciliary muscles change tension in supporting ligaments, changing lens curvature and focal length.
12.2 Near and distant focusing
For a distant object, incoming rays are nearly parallel. The eye lens is relatively thin and less strongly converging. For a nearby object, rays are more divergent and require stronger convergence, so the lens becomes thicker and more curved.
The syllabus mainly expects ray diagrams and corrective lenses, but this focusing mechanism explains why a healthy eye can place images of different object distances on the fixed retina.
12.3 Short-sightedness
In short-sightedness or myopia, distant rays are focused before the retina when the eye is relaxed. The eyeball may be too long or the optical system too strongly converging. Distant objects look blurred, while nearby objects may remain clear.
A diverging spectacle or contact lens spreads the incoming rays slightly before they enter the eye. The eye then converges them onto the retina. In a diagram, show the diverging lens making parallel rays appear to come from the person’s far point.
12.4 Long-sightedness
In long-sightedness or hyperopia, rays from a near object would focus behind the retina if the eye cannot accommodate sufficiently. The eyeball may be too short or the optical system too weak. Near work is difficult and may cause eye strain.
A converging lens begins the convergence before light enters the eye, allowing the eye to form the image on the retina. Age-related loss of accommodation, called presbyopia, is often corrected with converging reading lenses, though its cause differs from simple eyeball length.
12.5 Drawing correction diagrams
Draw the defective eye first, showing where rays would meet relative to the retina. Then place the corrective lens in front and show the final rays meeting on the retina. For short sight use a diverging lens; for long sight use a converging lens.
Do not simply label a lens without showing its effect on rays. The correction works by changing the vergence of light before it enters the eye, not by physically changing the retina position.
Worked examples
Choosing a lens
A student sees near objects clearly but distant objects are blurred. The eye focuses parallel rays before the retina, so the student is short-sighted and needs a diverging lens.
Long-sight diagram
Rays from a nearby object are not converged enough and would meet behind the retina. A converging lens increases convergence so they meet on the retina.
Practical focus
Investigation
Use an eye model with a water-filled lens or adjustable screen, if available. Change the model eye length to simulate defects and add converging or diverging lenses to restore a sharp image.
Examination guidance
- Short sight → image before retina → diverging correction. Long sight → image behind retina → converging correction.
- The image on the retina is real and inverted.
- Show ray paths, not only labels.
Check your understanding
- Which lens corrects myopia?
- Where does the image form in an uncorrected long-sighted eye?
- What changes during accommodation?
Answers
- A diverging lens.
- Behind the retina for the relevant object distance.
- The curvature and focal length of the eye lens change.
