The human eye, its defects and their correction
The eye lens forms a real, inverted image on the retina. In myopia far objects blur (concave lens); in hypermetropia near objects blur (convex lens).
Parts of the eye and their jobs
| Part | What it does |
|---|---|
| Cornea | The thin, clear bulge at the front of the eyeball. Light enters here, and most of the refraction happens at its outer surface |
| Iris | A dark muscular ring behind the cornea. It controls the size of the pupil |
| Pupil | The opening in the iris. It controls the amount of light that enters: small in bright light, wide in dim light |
| Eye lens | A clear, jelly-like convex lens. It fine-tunes the focal length so the image falls on the retina |
| Ciliary muscles | Hold the eye lens and change its curvature (how curved it is) |
| Retina | The light-sensitive screen at the back. Its cells make electrical signals when light falls on them |
| Optic nerve | Carries the signals from the retina to the brain, which makes sense of them |
How the eye focuses: power of accommodation
- The eye lens is a convex lens. It forms a real, inverted and smaller image on the retina.
- The distance from the lens to the retina is fixed. So the ciliary muscles change the curvature of the lens to focus near and far objects.
- A normal eye sees clearly from infinity (far point) down to 25 cm (near point). The 25 cm is the least distance of distinct vision.
- This ability to see clearly from infinity to 25 cm is the power of accommodation.
- Looking at a far object, the eye is relaxed. Looking at an object at 25 cm, the eye is under the most strain.
- An object looks bigger when it is nearer, because it makes a larger visual angle at the eye.
- The eye is most sensitive to yellow-green light of wavelength about 555 nm (5550 Å).
Myopia vs hypermetropia
These defects come from a change in the shape of the eyeball or of the eye lens. Myopia needs a minus lens; hypermetropia needs a plus lens.
Two more defects of vision
| Defect | What happens | Correction |
|---|---|---|
| Presbyopia | Comes with old age. The ciliary muscles lose their elasticity, so the eye loses its power of accommodation. Both near and far objects may blur. | Two pairs of spectacles, or one bifocal lens (concave part for distance, convex part for reading) |
| Astigmatism | The eye lens is not perfectly spherical, so its focal length differs in two directions at right angles. Lines in one direction look sharp and those across them look blurred. | A cylindrical lens set in the right direction |
How to draw myopia and hypermetropia with their correction
Draw four small figures. In each, draw the eye as a circle, with a lens near the front and the retina at the back. Draw a straight axis through the middle.
- 1Myopic eye: draw parallel rays from a distant object entering the eye. Make them meet at a point in front of the retina. Label it 'image in front of the retina'.
- 2Myopia corrected: draw a concave lens in front of the eye. The parallel rays spread out after it, as if they came from the far point O (dotted lines back to O). Now the eye lens brings them together on the retina.The concave lens has a focal length equal to the far point distance.
- 3Hypermetropic eye: mark N′, the normal near point, 25 cm in front of the eye. Draw rays from an object at N′ entering the eye. Make them meet at a point behind the retina.
- 4Hypermetropia corrected: draw a convex lens in front of the eye. Mark N, this eye's own near point, farther away than N′. Rays from N′ bend inwards after the lens, as if they came from N (dotted lines back to N). Now they meet on the retina.
- 5Label every figure: myopic or hypermetropic eye, retina, concave or convex lens, and the far point or near point.
Power of the corrective lens
(a) A myopic person's far point is 80 cm. Find the power of the lens that lets him see distant objects. (b) A hypermetropic person's near point is 100 cm. Find the power of the lens that lets her read a book held at 25 cm. (Take the lens to be close to the eye.)
- (a) The lens must make a very far object (1/u = 0) appear at the far point: v = −80 cm.
- 1/f = 1/v − 1/u = 1/(−80) − 0, so f = −80 cm = −0.80 m.
- P = 1/f = 1/(−0.80 m) = −1.25 D.
- (b) The lens must make a book at u = −25 cm appear at the near point: v = −100 cm.
- 1/f = 1/v − 1/u = −1/100 + 1/25 = (−1 + 4)/100 = 3/100, so f = 33.3 cm = 0.333 m.
- P = 1/f = 1/(0.333 m) = +3.0 D.
(a) A concave lens of power −1.25 D. (b) A convex lens of power +3.0 D.
A learner who must sit in the front row to read the board should have an eye test. Myopia is common and easily corrected. Many grandparents wear bifocals: the top part for distance, the lower part for reading.
1 marks: “A normal eye cannot see an object sharply if it is held closer than a certain distance. Name this distance and give its value.” The least distance of distinct vision (the near point of a normal eye): about 25 cm.
2 marks: “A person's spectacles have a power of −2 D. Name the defect, the kind of lens and its focal length.” Myopia (short sight). A concave lens. f = 1/P = 1/(−2) m = −0.5 m = −50 cm.
5 marks: “What are myopia and hypermetropia? With labelled diagrams, show each defect and how it is corrected.” Myopia: far objects blur; image forms in front of the retina; concave lens moves it back onto the retina (diagram). Hypermetropia: near objects blur; image forms behind the retina; convex lens brings it forward onto the retina (diagram). Label the eye, retina, lens and far or near point.
Myopia: minus lens (concave). Hypermetropia: plus lens (convex). Presbyopia: old age, bifocals. Astigmatism: cylindrical lens.