Chapter 10: The Human Eye and the Colourful World
1. The Human Eye
The human eye is one of the most valuable and sensitive sense organs. It works like a camera: it has a lens system that forms a real, inverted image of an object on a light-sensitive screen called the retina.
Main Parts of the Eye and Their Functions
| Part | Description / Function |
|---|---|
| Cornea | Thin transparent membrane forming the front bulge of the eyeball. Most of the refraction of light entering the eye occurs at the outer surface of the cornea. |
| Iris | Dark muscular diaphragm behind the cornea. It controls the size of the pupil (and hence the amount of light entering the eye). It gives the eye its colour. |
| Pupil | Small opening at the centre of the iris. It regulates and controls the amount of light entering the eye — it contracts in bright light and dilates (expands) in dim light. |
| Crystalline (eye) lens | Transparent, jelly-like convex lens made of fibrous material. It provides the finer adjustment of focal length needed to focus objects at different distances on the retina. |
| Ciliary muscles | Muscles holding the eye lens. By contracting or relaxing, they change the curvature and hence the focal length of the eye lens. |
| Retina | Delicate light-sensitive screen at the back of the eye containing enormous light-sensitive cells (rods and cones). The image formed here generates electrical signals. |
| Optic nerve | Carries the electrical signals from the retina to the brain, where the image is interpreted (erect). |
2. Power of Accommodation
Definition: The ability of the eye lens to adjust its focal length (by the action of ciliary muscles) so as to focus both near and distant objects clearly on the retina is called the power of accommodation of the eye.
- When we look at distant objects: ciliary muscles relax → lens becomes thin → focal length increases.
- When we look at nearby objects: ciliary muscles contract → curvature of lens increases → lens becomes thick → focal length decreases.
Least distance of distinct vision (Near point) of a normal young adult eye = 25 cm.
Far point of a normal eye = infinity.
So a normal eye can see clearly from 25 cm to infinity.
Cataract: In old age, the crystalline lens of some people becomes milky and cloudy. This causes partial or complete loss of vision. It can be treated by cataract surgery (removal/replacement of the cloudy lens), after which vision is restored.
3. Defects of Vision and Their Correction
(a) Myopia (Near-sightedness / Short-sightedness)
- The person can see nearby objects clearly but cannot see distant objects distinctly.
- The far point is nearer than infinity; the image of a distant object is formed in front of the retina.
- Causes: (i) excessive curvature of the eye lens (focal length too short), or (ii) elongation of the eyeball.
- Correction: A concave lens (diverging lens) of suitable power. It diverges the parallel rays from a distant object so that they appear to come from the person's far point; the eye then focuses them on the retina.
(b) Hypermetropia (Far-sightedness / Long-sightedness)
- The person can see distant objects clearly but cannot see nearby objects distinctly.
- The near point is farther than 25 cm; light from a nearby object is focused behind the retina.
- Causes: (i) focal length of the eye lens is too long, or (ii) the eyeball has become too small.
- Correction: A convex lens (converging lens) of suitable power. It provides additional converging power so that the image of a nearby object forms on the retina.
(c) Presbyopia
- Defect arising with ageing: the power of accommodation of the eye decreases, so the near point gradually recedes beyond 25 cm. Nearby objects appear blurred.
- Causes: gradual weakening of ciliary muscles and diminishing flexibility of the eye lens.
- Correction: convex lens for reading; a person who suffers from both myopia and hypermetropia uses bifocal lenses — the upper portion is concave (for distant vision) and the lower portion is convex (for near vision/reading).
| Defect | Cannot see | Image forms | Correcting lens |
|---|---|---|---|
| Myopia | Distant objects | In front of retina | Concave (diverging), negative power |
| Hypermetropia | Nearby objects | Behind the retina | Convex (converging), positive power |
| Presbyopia | Nearby objects (with age) | Behind the retina | Convex / bifocal lenses |
4. Refraction of Light Through a Prism
A glass prism is a transparent medium bounded by two plane refracting surfaces inclined at an angle. The angle of the prism (A) is the angle between its two lateral refracting surfaces.
- At the first surface (air → glass) the ray bends towards the normal; at the second surface (glass → air) it bends away from the normal.
- Unlike a rectangular glass slab (where the emergent ray is parallel to the incident ray), in a prism the emergent ray is bent towards the base of the prism.
Angle of deviation (∠D): the angle between the direction of the incident ray (produced forward) and the emergent ray. It measures how much the prism has deviated the light.
Diagram key points: Draw triangle ABC (prism), incident ray PE on face AB, refracted ray EF inside the prism bending towards the base, emergent ray FS from face AC. Mark angle of incidence (∠i), angle of refraction (∠r), angle of emergence (∠e), angle of prism (∠A) at the top, and angle of deviation (∠D) between incident direction produced and emergent ray.
5. Dispersion of White Light by a Glass Prism
Dispersion: The splitting of white light into its component colours on passing through a prism.
- The band of coloured components is called the spectrum: V I B G Y O R — Violet, Indigo, Blue, Green, Yellow, Orange, Red.
- Red deviates the least (appears at the top of the spectrum); violet deviates the most (bends most, near the base).
- Cause: different colours of light travel with different speeds in glass, so they bend through different angles — the prism disperses them.
- White light is not a single colour; it consists of seven colours. Any light producing a spectrum similar to sunlight is called white light.
Recombination (Newton's experiment): Newton placed a second identical prism inverted relative to the first in the path of the spectrum. The second prism recombined all the colours and a beam of white light emerged. This proved that sunlight is made up of seven colours and that the prism itself does not add colour.
Rainbow: A natural spectrum formed after rain by dispersion, internal reflection and refraction of sunlight by tiny water droplets in the atmosphere. Each droplet refracts and disperses the incident sunlight, reflects it internally, and refracts it again on the way out. A rainbow is always formed in the direction opposite to the Sun.
6. Atmospheric Refraction
Refraction of light by the Earth's atmosphere (due to air layers of gradually changing optical density/refractive index) is called atmospheric refraction.
- Twinkling of stars: Starlight is refracted continuously by the atmosphere whose physical conditions keep changing; the refractive index of air gradually increases towards the surface, so starlight bends towards the normal and the star appears slightly higher (apparent position) than its actual position. Since the atmosphere is not stationary, the apparent position and the amount of light entering the eye fluctuate — the star appears to twinkle.
- Why planets do not twinkle: Planets are much closer to the Earth and act as an extended source (collection of many point-sized sources); the total variation in light from all points averages out to zero, nullifying the twinkling effect.
- Advance sunrise and delayed sunset: Due to atmospheric refraction, the Sun is visible about 2 minutes before actual sunrise and remains visible about 2 minutes after actual sunset — the day appears about 4 minutes longer. The same refraction causes the apparent flattening (oval shape) of the Sun's disc at sunrise and sunset.
7. Scattering of Light
When light falls on very fine particles, it is scattered (redirected) in all directions. The colour of scattered light depends on the size of the scattering particles: very fine particles scatter mainly blue (shorter wavelength) light, while larger particles scatter light of longer wavelengths (and very large particles scatter all wavelengths, appearing white).
- Tyndall effect: The scattering of a beam of light by colloidal particles (smoke, dust, tiny water droplets, molecules of air) making its path visible. Examples: a beam of sunlight entering a smoke-filled room, sunlight passing through the canopy of a dense forest (mist droplets scatter the light).
- Blue colour of the sky: Air molecules and fine particles in the atmosphere are smaller than the wavelength of visible light and scatter blue light (shorter wavelength) much more strongly than red. The scattered blue light enters our eyes — the sky appears blue. If the Earth had no atmosphere, the sky would appear dark (as it does to astronauts/passengers at very high altitudes).
- Red at sunrise and sunset: Near the horizon, sunlight travels a much longer path through the atmosphere. Most of the blue and shorter wavelengths are scattered away; the light that reaches us is rich in red (longer wavelength), so the Sun and the surrounding sky appear reddish. At noon the path is shortest, so only a little light is scattered and the Sun appears white.
- Why danger signals are red: Red light is scattered the least by fog or smoke (longest wavelength in the visible region), so it is visible from the greatest distance.