Class 10 Science Notes Chapter 10 — The Human Eye and the Colourful World#
The human eye#
| Part | Function |
|---|---|
| Cornea | Thin transparent front membrane; most of the refraction happens at its outer surface |
| Iris | Dark muscular diaphragm that controls the size of the pupil |
| Pupil | Opening that controls how much light enters (small in bright light, large in dim light) |
| Eye lens | Fine-adjusts the focal length to focus on the retina; forms a real, inverted image |
| Ciliary muscles | Change the curvature, and so the focal length, of the eye lens |
| Retina | Light-sensitive screen with light-sensitive cells; sends electrical signals |
| Optic nerve | Carries signals from the retina to the brain, which sees the image upright |
- Power of accommodation — the eye lens’s ability to change its focal length.
- Distant object: ciliary muscles relax, the lens gets thinner, and its focal length increases.
- Near object: ciliary muscles contract, the lens gets thicker, and its focal length decreases.
- Near point (least distance of distinct vision) of a normal adult eye: 25 cm. Far point: infinity.
- Cataract — the lens becomes milky and cloudy, causing partial or complete loss of vision. It is treated by surgery.
Defects of vision#
| Defect | Can see clearly | Image forms | Causes | Corrected with |
|---|---|---|---|---|
| Myopia (near-sightedness) | Near objects only | In front of the retina | Eyeball too long, or lens too curved | Concave lens |
| Hypermetropia (far-sightedness) | Distant objects only; near point is beyond 25 cm | Behind the retina | Eyeball too short, or focal length too long | Convex lens |
| Presbyopia | Not near objects (with age) | Behind the retina | Ciliary muscles weaken and the lens loses flexibility | Convex lens; bifocal lens if both defects |
- A bifocal lens has a concave upper part (distant vision) and a convex lower part (near vision).
- Defects can also be corrected with contact lenses or surgery.
- Eye donation: one pair of eyes can give vision to up to four people with corneal blindness. Eyes must be removed within 4–6 hours of death.
Refraction through a prism#
- A prism has two triangular bases and three rectangular faces. The angle between two side faces is the angle of the prism.
- Light bends towards the base of the prism.
- Angle of deviation — the angle between the incident ray (extended) and the emergent ray.
Dispersion#
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Dispersion — splitting of white light into its colours by a prism.
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Spectrum — the band of colours: VIBGYOR (violet, indigo, blue, green, yellow, orange, red).
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Different colours bend by different amounts. Violet bends the most, red the least.
Dispersion: a prism splits white light into seven colours. Red bends the least and violet the most. -
Newton showed that a second, inverted prism recombines the colours into white light, so white light is made of seven colours.
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Rainbow — dispersion of sunlight by tiny water droplets in the air, which act like small prisms. The droplet refracts and disperses the light, reflects it internally, then refracts it again. A rainbow is always seen in the direction opposite to the sun.
Atmospheric refraction#
The atmosphere’s refractive index changes with temperature and height, so light bends as it passes through.
- Twinkling of stars: a star is a point source. Starlight bends by changing amounts, so its apparent position and brightness keep changing. Planets do not twinkle, because they are closer and act like a group of point sources whose changes cancel out.
- Stars look higher than they really are near the horizon.
- Advance sunrise and delayed sunset: we see the sun about 2 minutes before it actually rises and about 2 minutes after it actually sets.
- The flickering of objects seen through hot air above a fire is also atmospheric refraction.
Scattering of light#
- Tyndall effect — scattering of light by colloidal particles, which makes the path of a light beam visible (sunlight through a canopy of trees, a beam in a smoky room).
- The colour scattered depends on particle size. Very fine particles scatter mainly blue (shorter wavelength); larger particles scatter longer wavelengths.
| Observation | Reason |
|---|---|
| Sky is blue | Air molecules (smaller than the wavelength of visible light) scatter blue much more than red |
| Sky looks dark to astronauts in space | No atmosphere, so no scattering |
| Sun is reddish at sunrise and sunset | Light travels a longer path through the air; most blue is scattered away, so mainly red reaches us |
| Danger signals are red | Red is scattered least by fog and smoke, so it is seen from far away |
| Clouds are white | Large water droplets scatter all colours equally |
Common mistakes#
- Myopia → concave lens; hypermetropia → convex lens. Do not swap them.
- Saying red bends the most in a prism. Violet bends the most.
- Saying planets twinkle. Only stars do.
- Saying the sky is blue because of dispersion. It is because of scattering.