1. The human eye is one of the most valuable and sensitive sense organs, which nature has endowed on us to see the wonderful world of light and colour.
2. Our eyes have a lens system forming an inverted, real image on a light-sensitive screen inside the eye, called ‘retina’.
3. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
4. The front transparent bulge of the eye is called cornea. The cornea provides most of the refraction for the light rays entering the eye.
5. The crystalline lens provides the finer adjustment of focal length required to focus objects situated at different distances on the retina.
6. Iris controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye.
7. The retina is a delicate membrane, behind the crystalline lens, having an extremely large number of light-sensitive cells. These cells get activated upon illumination and generate electrical signals which are sent to brain via the optic nerves. The brain interprets these signals and processes the information so that we perceive objects as they are.
8. The eye lens is composed of fibrous, jelly-like material. Its curvature can be modified by the ciliary muscles and, consequently, the focal length (or power) of eye lens is changed.
9. When the ciliary muscles are relaxed, the eye lens becomes thin and its focal length is maximum and equal to the diameter of the eye ball. In this condition, one can see distant objects clearly.
10. At the time of looking at nearby objects the ciliary muscles of eye contract and eye lens becomes thicker. Consequently, focal length of eye lens decreases and nearby objects are clearly focussed at the retina.
11. Accommodation power is the property of the eye lens to adjust its focal length so as to focus objects situated at different distances from the eye on the retina.
12. Least distance of distinct vision or near point of an eye is the minimum distance from the eye, such that the objects situated there can be seen distinctly. For a young adult with normal vision the near point is at a distance of 25 cm.
13. The farthest point up to which an eye can see objects clearly, is called the far point of the eye. For normal vision, the far point of eye lies at infinity.
14. The impression of an object persists for about 1/16th of a second on the retina, even after the removal of the object. This property known as ‘the persistence of vision’ is made use of in motion-picture projection.
15. For some people in old age, the crystalline lens becomes hazy or even opaque due to the membrane development over it. It is called ‘cataract’ and causes partial or complete loss of vision. The defect can be removed by surgical method.
16. Sometimes, the eye of a person loses its power of accommodation and the person cannot see the objects distinctly and comfortably without strain on eyes. Such defects are called the defects of vision.
17. There are three common defects of vision which can be corrected by using suitable spherical lenses. These defects are (i) myopia, (ii) hypermetropia, and (iii) presbyopia.
18. In myopia or near-sightedness (or short-sightedness), a person can see nearby objects clearly but cannot see distant objects distinctly. For a myopic eye, the far point is not at infinity but has shifted nearer to the eye.
Myopia arises due to either (i) excessive curvature of the cornea, or (ii) elongation of the eyeball.
Myopia can be corrected by using a concave lens whose focal length has same numerical value as the distance of far point of the defective eye.
19. In hypermetropia or long-sightedness (or farsightedness), a person can see distant objects distinctly but cannot see nearby objects so clearly. For a longsighted eye, the near point is not at 25 cm but has shifted away from the eye.
Hypermetropia arises either because (i) the focal length of eye lens is too large, or (ii) contraction of the eyeball.
Hypermetropia can be corrected by using suitable convex lens, which forms virtual image of the object situated at 25 cm at the near point of defective eye so that now the eye lens can focus it on the retina.
20. The power of accommodation of the eye decreases with ageing due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. This defect is called ‘presbyopia’. Due to this defect, most people cannot read comfortably and distinctly and need convex (converging) lenses of appropriate power. Some persons suffer from both myopia and hypermetropia. They need bi-focal lenses.
In common type of bi-focal lenses, the upper portion of lens consists of a concave lens and facilitates distant vision. The lower portion of lens is a convex lens and facilitates near vision.
21. Nowadays defects of vision can be corrected by the use of contact lenses or through surgical technique (using laser light).
22. An optical prism has two triangular bases and three rectangular lateral surfaces, which are inclined to each other. Angle between its two lateral faces is called the angle of prism. Whenever refraction of light takes place through a prism, the emergent ray bends towards the base of the prism. The angle between the directions of incident ray and emergent ray is called the angle of deviation D (orδ).
23. When a beam of white light passes through a glass prism, it splits up into its constituent seven colours. These seven colours are remembered by the acronym ‘VIBGYOR’. The splitting of white light into its component colours is called ‘dispersion of light’. The band of the coloured components formed due to dispersion is called ‘spectrum’.
Dispersion is on account of different amount of deviations for light of different colours. Red coloured ray bends the least but violet ray bends the most. Thus, spectrum is formed. Seven colours of VIBGYOR can be recombined so as to form white light.
24. Rainbow is a natural spectrum formed in the sky after a rain shower. Tiny water droplets hanging in atmosphere act like small prisms. They refract and disperse the incident sunlight, then reflect it internally and finally refract it again and the light comes out of drop. Due to dispersion and internal refraction of light different colours reach the observer’s eye at different angles and hence rainbow is formed. Rainbow is always formed in a direction opposite to that of Sun.
25. Atmospheric air layer just near the earth surface is optically denser and upper layers of atmosphere are successively rarer and more rarer. Hence, a light ray passing through atmospheric air undergoes refraction. Since the physical conditions of air are not stationary, the apparent position of the distant object, as seen through the air, fluctuates. It is known as an effect of atmospheric refraction.
26. Light coming from a distant star on entering into the earth’s atmosphere gradually bends towards the normal on account of atmospheric refraction. Hence, the star appears slightly higher than its actual position when viewed near the horizon.
27. Since the physical conditions of the earth’s atmosphere are not stationary, the path of light rays coming from a star, which behaves almost as a point source of light, goes on varying slightly and it causes the twinkling of star.
As planets are much closer to the earth, hence average light coming from them in observer’s eye remains the same and hence planets do not twinkle.
28. On account of atmospheric refraction the Sun is visible to us 2 minutes before actual sunrise and for about 2 minutes after actual sunset. Thus, apparent duration of day (from sunrise to sunset) is increased by 4 minutes.
29. Scattering of light is spreading of light in different directions due to interplay of light with tiny particles like air molecules, dust particles, colloidal solutions etc.
Tyndall effect proves experimentally the phenomenon of scattering of light.
The colour of the scattered fight depends on the size of the scattering particles. Very fine particles scatter mainly blue violet light but particles of larger size may scatter light of longer wavelengths too.
30. Blue colour of clear sky is due to scattering of sunlight by air molecules and other fine particles in the atmosphere of earth.
In the absence of atmosphere no scattering is possible. As a result when seen from surface of moon, satellites and from planes flying at very high altitudes the sky appears dark.
31. Reddish colour of sun at sunrise or sunset, blue colour of water in deep sea are also on account of scattering effect.
32. Danger signal lights are of red colour because scattering of red light is least by fog or smoke. Hence, danger signal can be seen even from a longer distance.