The human eye: defects of vision and their correction
The eye lens changes its focal length to see near and far objects. Myopia is corrected with a concave lens and hypermetropia with a convex lens.
Parts of the eye
| Part | What it does |
|---|---|
| Cornea | Clear, curved front window of the eye; light enters through it |
| Iris | Coloured ring of muscle; controls the size of the pupil |
| Pupil | Hole in the centre of the iris; becomes small in bright light and large in dim light |
| Eye lens | Convex lens of clear, jelly-like material; focuses light on the retina |
| Ciliary muscles | Change the thickness of the lens, and so its focal length |
| Retina | Light-sensitive screen at the back; a real, inverted image forms here |
| Optic nerve | Carries signals from the retina to the brain, which sees the image erect |
Power of accommodation
The ciliary muscles can make the eye lens thinner or thicker. This changes its focal length. For a far object, the muscles relax and the lens becomes thin (longer focal length). For a near object, they contract and the lens becomes thick (shorter focal length). This ability is the power of accommodation. A normal eye sees clearly from the near point, 25 cm, to the far point, infinity. The near point distance is called the least distance of distinct vision.
Defects of vision
| Defect | Cannot see clearly | Cause | Image forms | Correction |
|---|---|---|---|---|
| Myopia (short sight) | Far objects | Eyeball too long, or focal length of the lens too short | In front of the retina | Concave lens |
| Hypermetropia (long sight) | Near objects | Eyeball too short, or focal length of the lens too long | Behind the retina | Convex lens |
| Presbyopia | Near objects, in old age | Ciliary muscles become weak and the lens loses flexibility | Behind the retina, for near objects | Convex lens for reading, or a bifocal lens (upper concave part for far vision, lower convex part for reading) |
| Astigmatism | Horizontal and vertical lines at the same time | Cornea not perfectly round: it curves differently in different directions | Not sharp in every direction | Cylindrical lens |
How to draw myopia, hypermetropia and their correction
- 1Draw a circle for the eyeball and a small double-convex lens at its front. Label eye lens and retina (the back of the circle).
- 2Myopia: draw parallel rays from a distant object entering the eye. Make them meet at a point in front of the retina. Label image.
- 3Myopia corrected: draw the same eye with a concave lens in front. The rays spread slightly after it and meet on the retina.
- 4Hypermetropia: draw rays from a near object meeting at a point behind the retina (mark it with dotted lines).
- 5Hypermetropia corrected: put a convex lens in front of the eye. The rays now meet on the retina.
Lenses for a myopic eye and a hypermetropic eye
(i) A person with myopia cannot see clearly beyond 2 m. What lens does the person need to see distant objects? Find its focal length and power. (ii) A person with hypermetropia has a near point of 1 m. Find the focal length and power of the lens needed to read a book at 25 cm.
- (i) The lens must make a distant object (u = −∞) appear at the far point, 2 m in front of the eye (v = −2 m).
- Lens formula: 1/f = 1/v − 1/u = 1/(−2) − 0 = −1/2.
- f = −2 m. The negative sign shows a concave lens.
- Your textbook's formula gives the same: f = xy/(x − y), where x is the far point and y the distance the person wants to see. For distant objects (y = ∞), f = −x = −2 m.
- Power: P = 1/f = 1/(−2) = −0.5 D.
- (ii) Your textbook's formula: f = dD/(d − D), where d = 1 m is the near point of the eye and D = 0.25 m.
- f = (1 m × 0.25 m) ÷ (1 m − 0.25 m) = 0.25 ÷ 0.75 = 1/3 m ≈ 0.33 m. The positive sign shows a convex lens.
- Power: P = 1/f = 1 ÷ (1/3 m) = +3 D.
(i) A concave lens of focal length 2 m and power −0.5 D. (ii) A convex lens of focal length about 33 cm and power +3 D.
A student on the last bench who cannot read the blackboard may have myopia, so school eye tests matter. Vitamin A from carrots, papaya and green leafy vegetables keeps the retina healthy and prevents night blindness.
2 marks: “What is the power of accommodation of the eye?” The ability of the eye lens to change its focal length, using the ciliary muscles, to see both near and far objects clearly.
4 marks: “What is myopia? Give its causes and correction, with diagrams.” Far objects look blurred; eyeball too long or focal length too short; image forms in front of the retina; corrected with a concave lens; two ray diagrams.
7 marks: “Explain myopia and hypermetropia with their causes and correction. Draw diagrams.” Myopia: far objects blurred, long eyeball, image in front of retina, concave lens. Hypermetropia: near objects blurred, short eyeball, image behind retina, convex lens. Four labelled diagrams.
Myopia: Minus lens (concave). Hypermetropia: plus lens (convex). A short-sighted person sees only short distances clearly.