THE HUMAN EYE
The construction and working of the human eye is similar to photographic camera in many respects. Human eye is almost a spherical ball, with a light bulge in the front. The structure and function of each part of the eye is given below :
- Sclerotic: It is the outermost covering of the eye ball. It is made of white tough fibrous tissues. Its function is to house and protect the vital internal parts of the eye.
- Cornea: It is the front bulging part of the eye. It is made of transparent tissues. Its function is to act as a window to the world, i.e., to allow the light to enter in the eye ball.
- Choroid: It is a grey membrane attached to the sclerotic from the inner side. Its function is to darken the eye from inside and, hence, prevent any internal reflection.
- Optic Nerve: It is a bundle of approximately 70,000 nerves originating from the brain and entering the eye ball from behind. Its function is to carry optical messages (visual messages) to the brain.
- Retina: The optic nerve on entering the ball, spreads like a canopy, such that each nerve end attaches itself to the choroid. The nerve endings form a hemi-spherical screen called retina. These nerve endings on the retina are sensitive to visible light. On retina there are two important areas namely yellow spot and Blind spot. The function of retina is to receive the optical image of the object and then convert it to optical pulses. These pulses are then sent to the brain through optic nerve.
- Yellow spot: It is a small area, facing the eye lens. It has high concentration of nerve endings and is slightly raised as well as slightly yellow in colour. Its function is to form a very clear image by sending a large number of optical pulses to brain.
- Blind Spot: It is a region on the retina, where the optic nerve enters the eye ball. It has no nerve ending and hence, is insensitive to light. It does not seem to have any function. Any image formed on this spot is not visible.
- Crystalline lens: It is a double convex lens made of transparent tissues. It is held in position by a ring of muscles, commonly called ciliary muscles. Its function is to focus the images of different objects clearly on the retina.
- Ciliary Muscles: It is a ring of muscles which holds the crystalline lens in position . When these muscles relax, they increase the focal length of the crystalline lens and vice versa. Its function is to alter the focal length of crystalline lens so that the images of the objects, situated at different distances, are clearly focused on the retina.
- Iris: It is a circular diaphragm suspended in front of the crystalline lens. It has a tiny hole in the middle and is commonly called pupil. It has tiny muscles arranged radially around the pupil. These muscles can increase or decrease the diameter of the pupil. The iris is heavily. pigmented. The colour of eyes depends upon colour of pigment.
The function of iris is to control the amount of light entering the eye. This is done by increasing or decreasing the diameter of pupil
- Vitreous Humour: It is a dense jelly-like fluid, slightly grey in colour, filling the part of eye between crystalline lens and retina. Its function is (i) to prevent the eye ball from collapsing due to change in atmospheric pressure, (ii) in focusing the rays clearly on the retina.
- Aqueous Humour: It is a watery, saline fluid, filling the part of the eye between the cornea and the crystalline lens. Its function is (i) to prevent front part of the eyeball from collapsing with the change in atmospheric pressure, (ii) to keep the cornea moist.
POWER OF ACCOMMODATION OF THE EYE
When the object is infront of eye then light rays coming from the object are refracted and after passing through the vitreous humour they are focused on the retina. The sensation from retina is conveyed to the brain through the optic nerves and the object becomes visible.
When eye is in normal condition i.e. the muscles are not strained then a clear image of the objects situated at infinity is formed on the retina. This is possible only when the distance between lens and retina is equal to the focal length of the lens(fig). When the object is close to the eye then its image should be formed behind the retina and it should by blurred, but it is not so in reality. Because when the object is close to eye then muscles are strained automatically. Muscles get contracted to make lens thicker at the centre which reduces the focal length of the lens and image is again formed on the retina. The ability of self adjustment of focal length of the eye lens is called accommodation power.
DEFECTS OF VISION AND THEIR CORRECTION
Abnormalities in the normal vision of the eye are called defects of vision or defects of eyes. The most commonly observed defects of vision (or defects of eyes) are:
(i) Myopia or shortsightedness or nearsightedness
(ii) Hypermetropia or longsightedness or hyperopia or farsightedness (
Short Sightedness (or myopia)
Shortsightedness (or myopia) is the defect due to which the eye is not able to see the distant objects clearly though it can see the nearby objects clearly. So, a shortsighted or myopic eye has its far point nearer than infinity.
What causes shortsightedness (or myopia)
Myopia or shortsightedness is caused by the following reasons.
(a) Decrease of focal length of the eye lens, i.e. the eye lens becomes more convergent.
(b) Elongation of the eyeball, i.e. the increased length of the eyeball.
How is shortsightedness (or myopia) corrected?
The shortsightedness (myopia) can be corrected by making the eye lens less convergent. This can be done by placing a concave lens (divergent lens) of suitable focal length before the eye lens. The rays of light coming from a distant object after passing through the concave (diverging) lens of the spectacles diverge slightly. As a result, the rays entering the eye appear to come from the far point of the myopic eye, and therefore get focused at the retina to form a clear image.
How to calculate the focal length and power of the lens used for correcting a myopic eye
The corrective lens (concave lens) needed to correct a myopic eye should form the image of the far-off object (e.g. at infinity) at the far point (d) of the myopic person.
Thus, u = – ∞ , v = –d, f = ?
∴ ⇒ f = -d
Longsightedness (or hypermetropia or hyperopia)
The longsightedness (or hypermetropia) is the defect due to which the eye is not able to see clearly the nearby objects though it can see the distant objects clearly. So, a longsighted eye has its near point farther away from the normal near point (about 25 cm for an adult).
What causes longsightedness (or hypermetropia)
Hypermetropia or longsightedness is caused due to the following reasons:
(i) Increase of the focal length of the eye lens, i.e. the eye lens becoming less convergent.
(ii) Shortening of the eye ball, i.e. the length of the eye ball has decreased.
How is longsightedness (or hypermetropia) corrected
Longsightedness (hypermetropia) can be corrected by making the eye lens more convergent. This is generally done by placing a convex lens (converging lens) of suitable focal length before the eye lens. This is shown in Fig. The rays from a nearby object (about 25 cm) after passing through the convex lens of the spectacles converge slightly. As a result, the rays entering the eye appear to come from the near point of the longsighted eye, and therefore get focused at the retina to form a clear image. How to calculate the focal length and power of the lens used for correcting a hypermetropic eye The corrective lens (a convex lens) needed to correct a hypermetropic (or longsighted) eye should form the image of the object placed at the normal near point (the least distance of distinct vision is 25 cm) at the near point of the hypermetropic person. Thus,
v = Near point distance of the hypermetropic eye = – d
u = Near point distance for the normal eye = – D = – 25 cm Using the lens formula,
This defect arises due to different sections of cornea having different radii of curvature. One section of cornea may be more sharply curved that the other. The man cannot focus on both horizontal and vertical line simultaneously. For remedy a cylindrical lens with the axis of the cylindrical lens parallel to the correct axis of the cornea, is used.
If along with astigmatism, myopia or hypermetropia is also associated, which is generally very common, then for the complete remedy of the defect, sphero-cylindrical (or compound) lens are used. Eye with this defect is unable to see the lines in different axes but at the same distance with same clarity. It occurs due to irregular curvature of cornea/by birth or arises due to some injury. Horizontal and vertical lines can’t be seen simultaneously with this defective eye. Object in one direction get well focused and in perpendicular direction remain blurred. Correction: In this case, the spectacles are cylindrical lenses of suitable focal length.
This defect is usually found in older persons. Due to stiffening of ciliary muscles, controlling the curvature of the lens reduces, thus the eye loses much of its accommodating power. As a result distant as well as nearby objects cannot be seen clearly in proper perspective. That is, in this defect near point as well as far point of the eye is affected. For the remedy of this defect two separate lenses or one bifocal lens is used. For visualizing nearby objects, convex lens is used, where as for seeing distant objects concave lens is used.
A homogeneous solid transparent and refracting medium bounded by two plane surfaces inclined at an angle is called a prism:
Refraction through a prism:
(a) PQ and PR are refracting surfaces.
(b) ∠QPR = A is called refracting angle or the angle of prism (also called Apex angle.)
(c) δ = angle of deviation
(d) For refraction of a monochromatic (single wave length) ray of light through a prism;
δ = (i + e) – (r1 + r2) and r1 + r2 = A
∴ δ = i + e – A.
DISPERSION OF WHITE LIGHT
Sir Isaac Newton, while working with an astronomical telescope, observed that the images of stars as seen through the telescope were coloured near the fringes. He got the lenses of the telescope polished, but found that the colour still persisted. From the above observation, he concluded that the fault may not be with the lenses, but it had something to do with the nature of light itself. To investigate this conclusion, he performed the following experiment.
Experiment: Newton allowed sunlight to enter through a small hole in a window of a darkened room. He placed an equilateral prism in the path of the narrow beam of light. The light emerging from the prism was allowed to fall on the white screen. It was found that light received on the white screen was a band of seven colours. The order of colours from the base of prism is violet, indigo, blue, green, yellow, orange and red. This order of colours can be easily remembered by remembering the word VIBGYOR.
(a) Dispersion: The phenomenon due to which white light splits into seven colours (VIBGYOR), when passed through an equilateral prism is called dispersion.
(b) Spectrum: The band of seven colours obtained on the screen, when white light splits into seven colours is called spectrum.
What is meant by monochromatic and polychromatic light
The light of one single colour, or of one single wavelength is called monochromatic light (chrome means colour). Sodium light is golden yellow in colour. So, sodium light is monochromatic light. The light made up of many colours, or light consisting of radiations of many wavelengths, is called polychromatic light. White light is made up of seven colours. So, white light (or sunlight) is a polychromatic light.
How is the dispersed white light recomposed
Recombination of the seven colours of the dispersed white light to get white light is called recomposing of the dispersed white light.
How does a rainbow form
Rainbow is an example of the dispersion of white light. Just after the rain, a large number of small droplets of water remain suspended in the air. Each drop acts like a small prism. When sunlight falls on these drops, the white light splits into seven colours. The dispersed light from a large number of drops forms a continuous band of seven colours. This coloured band is called rainbow. Thus, rainbow is produced due to dispersion of white light by small raindrops hanging in the air after the rain. The rainbow is seen when the sun is behind the observer.
Scattering of light
When light falls on tiny particles then diffused reflection takes place and light spreads in all possible direction. This phenomenon is known as scattering of light. Small particles scatter mainly blue light. When size of the particle increases then the light of longer wavelength also scatter. The path of a beam of light passing through a true solution is not visible. However, its path becomes visible through a colloidal solution where the size of the particles is relatively larger. Rayleigh proved that the intensity of scattered light is inversely proportional to the fourth power of the wavelength, provided the scatters is smaller in size than the wavelength of light:
(a) Tyndall Effect:
The earth’s atmosphere is a heterogeneous mixtures of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particle gives rise to tyndall effect. This phenomenon is seen when a fine beam of sunlight enters a smoke filled room through a small hole. Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.
(b) Phenomena based upon Scattering of Light:
A number of optical phenomena can be explained on the basis of scattering of light:
(i) Colour of the clear sky is blue: When we look at the sky, we receive sunlight scattered by fine dust particles, air molecules and water-vapour molecules present in the atmosphere. Since blue light, which is present in larger proportion of the violet light in the sunlight, is scattered about ten times more than the orange-red light, the light reaching the eye is mainly blue. Hence the sky appears bluish. If the earth had no atmosphere, there were no scattered sunlight and the sky would have appeared black. In fact, the sky does appear black to the astronauts in the space above the earth’s atmosphere.
(ii) The clouds appears white: The dependence of scattering on 1/l4 is valid only when the scatterer particles or molecules are much smaller than the wavelength of light, as are air molecules. Clouds, however, contain water droplets or ice crystals that are much larger than l and they hence scatter light of all wavelengths nearly equally. Hence clouds appear white.
(iii) At sunrise or sunset the sun appears reddish: The scattering of light also explains the reddish appearance of sun at sunrise or sunset. At sunrise or sunset, the sun is near the horizon and the sunrays reach the earth after passing through a maximum distance in the atmosphere. During this passage, the light is scattered by air molecules and fine dust particles. Since scattering µ 1/l4, most of the blue and neighbouring coloured light is scattered out before reaching the observer. Hence the light received by the observer is predominantly red. (For a similar reason, the sun appears orange-red in fog or mist.) At noon, when the sun is overhead, the sunrays travel minimum distance in the atmosphere and there is little scattering. Hence the sun appears almost while (infact, slightly yellowish because some blue light is scattered).
(C) Experimental verification of Scattering:
Let us do an activity to understand the colour of sun at sunrise and sunset. Place a strong source (s) of white light at the focus of converging lens (L1). This lens provides a parallel beam of light to pass through a transparent glass tank (T) containing clear water. Allow the beam of light to pass through a circular hole (C) made in a cardboard. Obtain a sharp image of the circular hole on a screen (MN) using a second converging lens (L2).
Dissolve 200g of sodium thiosulphate in 2L of clear water taken in the tank. Add 1 to 2 mL of concentrated sulphuric acid to the water. We observe that microscopic sulphur particles precipitate in 2 to 3 minutes. As sulphur particles begin to form we can observe the blue light from the three sides of the glass tank. It is due to scattering of short wavelengths by minute colloidal sulphur particles. We observe that the colour of the transmitted light from the fourth side of glass tank facing the circular tank at first is orange red colour and then bright crimson red colour on the screen. Light from the sun near the horizon passes through thicker layer of air and larger distance in the earth’s atmosphere before reaching our eyes. Light from the sun travel relatively short distance. At moon, the sun appears white.
As a little of blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelength are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelength. This gives rise to the reddish appearance of the sun.
Use of multiple reflections
The keleidoscope is a device that uses reflections to produce patterns. It consists of mirrors inclined to each other. The mirrors form multiple images of objects in front of them. This creates beautiful patterns, which change when the keleidoscope is rotated or shaken.
The working of a periscope is based on the principle of successive reflections from two plane mirrors. It consists of two plane mirrors M1 and M2 facing each other fixed at 45° to the framework of a tube which is bent twice at right angle (fig a). A beam of light from some object is turned through one right angle by the mirror M1. In the same way the light is deviated through another right angle by the mirror M2. Therefore, the object is seen by the eye in spite of the obstacle. This arrangement can be used by a person to see a match over the heads of a few people while standing at the back of the crowd.
Even an object can be seen through a wall as well by an arrangement as shown in fig.(b) In this case, light from the candle is reflected by four mirrors M1, M2, M3 and M4 before reaching the eye. Therefore, the candle is seen through the wall.
COLOURS OF THE OBJECT
(a) Colour of objects in White and Coloured Light: We known that white light is a mixture of seven colours. Light can be of different colours. Let us understand that why different objects appear to have different colours. A rose appear red because when white light falls on rose, it reflects only the red component and absorbs the other components. We conclude that the colour of an object depends upon the colour of light it reflects.
(i) If an object absorbs lights of all colours and reflects none, it appears black.
(ii) If an object reflects light of all colour, it appears white when seen in white light.
(iii) When we talk of colour of an object, we refer to its colour as seen in white light.
(iv) A rose will appear black in green light because there is no red component in the light and it will not reflect any light. Hence no light will come from rose to the eye. Similarly if a green leaf is seen in red light, it appears black.
(v) If a white flower is seen in red light, it appear red because a white object reflects light of all colours falling on it. So it reflects the red light falling on it, which then enters the eye.
(b) Primary Colours of Light: Red, green and blue are primary colours of light and they produce white light when added in equal proportions. All colours can be obtained by mixing these three colours in different proportions.
(c) Secondary Colours or Composite Colours of Light: The colours of light produced by adding any of primary colours are called secondary colours. Cyan, magenta and yellow are secondary colours of light.
Red + Green = Yellow
Green + Blue = Cyan
Red + Blue = Magenta
The method of producing different colours of light by adding the primary colours is called colour addition.
(d) Complementary Colours of Light: The lights of two colours which when added in equal proportions produce white light are called complementary colours of light and the two colours are called complements of each other.
For example, yellow and blue light are complementary colours of light because when they mixed in equal proportions, they produce white light. We can also find the pairs of complimentary colours of light as follows.
(Red + Green) + Blue = Yellow + Blue = White
Red + (Green + Blue) = Red + Cyan = White
(Red + Blue) + Green = Magenta + Green = White
The above results can be diagrammatically represented in the form of a triangle as shown in figure. The outer limbs of the figure show the results of the addition of primary colours red, green and blue. The complementary colour pairs such as red and cyan are opposite to each other.
(e) Primary Colours of Pigment: Pigments are those substances that give colour to an object. The colour of a pigment as seen by us depends on what components of light it absorb or subtract from white light before reflecting that rest to our eyes. A primary colour (cyan, magenta, yellow) of a pigment is due to a primary colour of light being subtracted from white light.
White – Red – Blue + Green = Cyan
White – Green = Red + Blue = Magenta
White – Blue = Red + Green = Yellow
Mixing CMY (cyan, magenta, yellow) pigment in the correct proportions can produce millions of colour. If equal amount of pure
CMY pigments are mixed, we should get a black pigment. However, printers use black ink in addition to CMY inks to get good results.