0Human Eye And The Colourful World:
Human eye resembles a camera. It has a lens screen, ciliary muscles, cornea etc. Their functions are as follows:
- Cornea: Thin membrane of eye through which light enters the eye first and gets refracted.
- Iris: It is a dark muscular diaphragm that controls the size of the pupil.
- Pupil: Small hole in the iris, appears black because no light is reflected from it. Pupil regulates and controls the amount of light entering the eye.
- Eye lens: Transparent crystalline lens, made up of fibers, jelly like material, convex in nature.
- Ciliary muscles: It holds the lens and can modify the lens curvature.
- Retina: It is a screen in the eye, delicate membrane with lot of light sensitive cells called rods and cones. These cells get activated on illumination and produce electrical signals.
- Optic nerve: It is a nerve connecting eye and brain. It sends the electrical signals to brain. Diameter of human eye is approx. 2.3 cm.
Power of Accommodation: The ability of the eye lens to see far and near objects by adjusting its focal length. It occurs through following mechanism:
(a) Relaxation of ciliary muscles: Lens becomes thin, focal length of eye ‘f’ increases. One can see distant objects.
(b) Contraction of ciliary muscles: Lens becomes thick, focal length of eye ‘f ’ decreases. One can see near objects.
Least distance of distinct vision (near point D): The minimum distance at which objects can be seen most distinctly without strain.
For normal vision in young adult D = 25 cm.
Far Point: The farthest point (for normal vision it is at infinity) up to which the eye can see objects clearly.
Persistence of vision: The image formed on retina remains for about 1/10th of a second and is called persistence of vision.
Used in cinematography: Sequence of showing 24 images or more per second, appears to be in motion.
Light sensitive cells:
| Rods
| Cones |
| 1. Respond to intensity of light. | 1. Respond to colour. |
| 2. Enables to see in dim light. | 2. Become active in bright light. |
| 3. Cannot distinguish various colours. | 3. It can distinguish between various colours. |
Colour blindness: It is a genetic disorder caused due to absence of cone cells in the retina. Vision is normal but eye cannot distinguish between colours.
**People with colour blindness are not issued driving license**
Retina of chicks (birds) have mostly cones and few rods. So the chick wakes up with sunrise and sleeps by sunset.
Cataract: The crystalline lens of some old people becomes hazy or even opaque due to the development of a membrane over it, which leads to the loss of vision of eye. It can be corrected by surgery.
Defects of vision, cause and remedy:
- Refraction of light through a prism: - Prism has two triangular bases and three rectangular lateral surfaces. The surfaces are inclined to each other. The angle between two lateral faces is called the angle of prism. Incident ray enters the prism, gets refracted, it bends towards the normal and when it moves out of the prism, it now bends away from the normal and forms emergent ray. The peculiar shape of the prism makes, the emergent ray bend at an angle to the direction of the incident ray. This angle formed is called angle of deviation.
- Dispersion: The splitting of white light into its components due to different bending ability for colours when it pass through prism is called dispersion. Dispersion of white light.
- Spectrum: The band of seven colours obtained due to splitting of light is called spectrum. The red colour light bends the least and the violet colour light bends the maximum.
- Rainbows: Rainbow is a natural spectrum that appears in the sky after rain shower. The water droplet present in the atmosphere splits light.
- Atmospheric Refraction: Refraction of light by the Earth’s atmosphere.
- Twinkling of Stars : The light of star when enters Earth’s atmosphere undergoes refraction, the light bends towards the normal as light travels from rarer to denser medium so the position of star spotted in sky is different from its actual position. Star is a point sized source of light and the amount of light entering air changes. Sometimes it appears brighter and at some time hazier as the light travels from far distant and the atmosphere is not same always. Sun is visible 2 minutes before actual sunrise and about 2 minutes after the actual sunset because of atmospheric refraction.
- Sky appears blue: The small sized particles of air scatter blue light (shorter wavelength) that enter our eyes.
- Sky appears black: When there is no atmosphere (space) or no light (night), the scattering does not take place.
- Sky appears red: During sunrise/sunset the sun is near the horizon, light passes through larger distance in the earth’s atmosphere. Most of the blue light is scattered away. The red light does not get scattered and reaches our eyes.
