CHAPTER-11
The Human Eye & the Colourful World
[1] The Human Eye [2] Defects of vision & their correction [3] Refraction of light through a Prism [4] Atmospheric refraction [5] Scattering of Light [6] Lens Power & Lens Formula.
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[1] The Human Eye
1.1 Eye is one of the most sensitive and integral sense organs of human being. It is the most significant organ as it enables to see the world. It is important to discuss about the structure, functioning, its parts and its defects.
1.2 Human eye has many parts which function altogether and enable to see anything visible. These most important parts are discussed in successive paras hereinafter.
1.3 Retina: Retina is the sensory membrane that lines the inner surface of the back of the eyeball. It is located near the optic nerve. The purpose of Retina is to receive light that the lens has focused, convert it into neural signals, and send these signals to the brain for visual recognition.
1.4 Cornea: Cornea is the transparent front part of eye that covers the Iris, Pupil, and anterior chamber. Cornea, with the anterior chamber (the fluid-filled inside of the eye). Its main function to refract or bend light. Cornea is responsible for focusing most of the light that enters into the eye. Cornea accounting for approximately two-thirds of the eye’s total optical power.
1.5 Iris: Iris is a thin, annular structure in the eye responsible for controlling the diameter and size of the Pupil and hence the amount of light reaching the retina. Eye colour is defined by that of the Iris. In optical terms, the Pupil is the eye’s aperture, while the Iris is the diaphragm.
1.6 Pupil: Pupil is the opening in the centre of the Iris. The function of Pupil is to allow light to enter into the eye so that it can be focused on the retina to begin the process of sight.
1.7 Functioning of human eye: The eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles. The change in the curvature of the eye lens can thus change its focal length. When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases. This enables one to see distant objects clearly. When one is looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker. Consequently, the focal length of the eye lens decreases. This enables to see nearby objects clearly. The ability of the eye lens to adjust its focal length is called accommodation. However, the focal length of the eye lens cannot be decreased below a certain minimum limit. To see an object comfortably and distinctly, one must hold it at about 25 cm from the eyes. The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye. For a young adult with normal vision, the near point is about 25 cm. The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye. It may be noted here that a normal eye can see objects clearly that are between 25 cm and infinity. Sometimes, the crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
[2] Defects of vision & their correction
2.1 There is a situation when eye may gradually lose its power of accommodation. In these conditions, the person cannot see the objects distinctly and comfortably. The image/vision becomes blurred due to the refractive defects of the eyes.
2.2 There are primely three common refractive defects of vision – (a) Myopia or Near-sightedness or Short-Sightedness, (b) Hypermetropia or Far-sightedness or Long-Sightedness & (c) Presbyopia.
2.3 Myopia: In this type of defect, a person can see nearby objects clearly but cannot see distant objects distinctly. This defect may arise due to excessive curvature of eye lens, or elongation of the eyeball.
2.4 In this defect, a person has the far point nearer than infinity and the image is formed in front of Retina not on the Retina.
2.5 To correct this defect, a concave lens of suitable power is used and this enable the eye to form the image on retina.
2.6 Hypermetropia: In this type of defect, the person can see distant objects clearly but cannot see nearby objects distinctly. This defect may arise due to – (a) the focal length of the eye lens is too long, or (b) the eyeball becomes too small.
2.7 In this defect, a person has the near point far away (beyond 25cm) from the normal near point (25cm). As a result, the image forms behind Retina in place of on Retina.
2.8 To correct this defect, a convex lens of suitable power is used and this enable the eye to form the image on retina.
2.9 Presbyopia: When eyes gradually lose the ability to see things clearly up close. In this defect, the near point gradually recedes away. It is a normal part of aging. In fact, the term “presbyopia” comes from a Greek word which means “old eye.” Presbyopia may be experienced shortly after 40 years of age.
2.10 This defect causes due to – (a) the gradual weakening of the ciliary muscle & (b) diminishing flexibility of the eyes lens. This is corrected by using corrective eye-glasses.
2.11 To get rid of this defect, bi-focal lens spectacle is used. This defect is also cured by surgical interventions.
Questions (Page 190)
Q-1: What is meant by power of accommodation of the eye?
Ans: The ability of eye lens to adjust its focal length to clearly focus rays coming out from objects which are kept at distant & near to eyes on the retina is known as the power of accommodation of the eye.
Q-2: A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens used to restore proper vision?
Ans: An individual with a myopic eye should use a concave lens of focal length 1.2 m so that proper vision of the person can be restored.
Q-3: What is the far point and near point of the human eye with normal vision?
Ans: The minimum distance of an object from the eye, which can be seen distinctly without strain is called the near point of the eye. For a normal person’s eye, this distance is 25 cm.
The far point of the eye is the maximum distance to which the eye can see objects clearly. The far point of a normal person’s eye is infinity.
Q-4: A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Ans: The student is suffering from short-sightedness or myopia. Myopia can be corrected by the use of concave lens of an appropriate power.
[3] Refraction of light through a Prism
3.3 Prism: Prism is a solid three-dimensional figure whose base and two upper surfaces are rectangular & two surfaces are triangular in shape. A prism has – (a) Three Rectangular surfaces & (b) Two Triangular surfaces.
3.2 In the picture of Prism – (a) Sides/surfaces ABFD, ADEC & BCEF are three rectangular sides (b) Sides ABC & DEF are two triangular sides/surfaces.
3.3 Refraction thorough prism: To consider and examine the refraction of light through a Prism. It is considered that a ray of light PQ is incident upon the face AB of Prism ABC. Ray PQ strikes Prism surface AB at point E and deviates with an angle and bends towards the nomal (imaginary line perpendicular to AB) say NEN’ and again strikes the other surface AC of the Prism ABC
and again deviates with a angle and passes through in FRS direction. The picture diagram is self-explanatory.
Whenever a light ray enters from a rare medium (air) to a denser medium (glass/water) it bends towards Normal (the imaginary line perpendicular to the incidence surface) and when light ray passes through denser medium (glass/water) to rare medium (air), it goes away from the Normal.
Angle of incidence (<i) = Angle of refraction (<r).
3.4 Dispersion of white light by a glass prism: When a white light is passes through a glass prism it splits into seven different colours in a fixed sequence of colours which are – (i) Violet, (ii) Indigo (iii) Blue (iv) Green (v) Yellow (vi) Orange & (vii) Red. The splitting of light into its component colours is called ‘dispersion’. This happens because white light is a composition of seven colours and since every colour has its own wave length hence they deviate with specific different angle and as a result every separate colour is visible. Sir Isaac Newton was the first to use glass prism to obtain the spectrum of sunlight. Rainbow is the example of dispersion of light.
3.5 Sir Isaac Newton again tried to split the colours of the spectrum of white light 
further by using another similar prism by placing an identical prism in an inverted position with respect to the first prism and found that when the colours of spectrum pass through the second prism it again combined and emerged as a white light. This observation gave the idea that the sun light is made up of seven colours. Hence, any light that gives a spectrum similar to that of sunlight is often taken as white light.
[4] Atmospheric refraction
4.1 The refraction of light by the earth’s atmosphere is called atmospheric refraction. It happens above a fire or a radiator.
4.2 The air above fire are hot and hot air are a less dense medium than cooler air. Owning to refraction of light, the vision through hot air area appears fluctuating because the physical condition of the refracting medium (air) is not stationary. Hence, the apparent position of the object as seen through the hot air fluctuates.
4.3 Twinkling of stars: The star light on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. Since, the physical conditions of the earth’s atmosphere are not stationary and hence, the density of the atmosphere changes and star light keeps refracting due to change in density of atmosphere (air) accordingly. Hence, it appears twinkling as starlight entering the eye flickers.
4.4 Advance sunrise and delayed sunset: The Sun is visible to about 2 minutes 
before the actual sunrise and about 2 minutes after the actual sunset due to atmospheric refraction. By actual sunrise, it means the actual crossing of the horizon by the Sun. The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.
[5] Scattering of Light
5.1 ‘Scattering of light’ is the phenomenon in which light rays get deviated from its straight path on striking an obstacle like dust, gas molecules, water vapours etc.
5.2 The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then the scattered light may even appear white.
5.3 Scattering of light gives rise to many spectacular phenomena such as ‘Tyndall effect’ and the ‘red hues of sunrise and sunset’ etc.
5.4 The atmosphere of the earth is a heterogeneous mixture of minute particles. These particles include – (a) smoke, (b) tiny water droplets, (c) suspended particles of dust and (d) molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches to earth, after being reflected diffusely by these particles.
5.5 Tyndall Effect: It is a phenomenon in which the particles in a colloid scatter the beams of light that are directed at them and the path of light beam becomes visible.
5.6 Colour of clear sky: The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters into eyes. Hence, clear sky appears blue. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights. It is observed that ‘danger’ signal lights are red in colour because the red is least scattered by fog or smoke. Therefore, it can be seen inn the same colour at distance.
5.7 Colour of sunset & sunrise: At the time of sunset and sunrise, the sun is at the horizon of the earth and as a result, sun lights travel long distance to reach to the earth surface.
Light from the sun near the horizon passes through a thicker layer of air and larger distance in the earth’s atmosphere before reaching to the visitor.
Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Hence, the light that reaches to the earth surface is of longer wavelength. This gives rise to the reddish appearance of the sun.
[6] Miscellaneous
6.1 Defects of eyes vision are corrected by using appropriate lens of suitable power as per kind of defects. Hence, it is necessary to discuss how the suitable power of lens is determined for a correct solution of vision defect of an eye.
6.2 Power of optical lens: The power of optical lens is defined as the reciprocal of focal length. Power of an optical lens is measured in Diopter and it is denoted by ‘D’.
If focal length is ‘f’ and power of lens is ‘P’ then the relationship between ‘f’ & ‘P’ is
P = 1/f (in meter).
6.3 Convex lens which is also known as converging lens have positive focal length. Hence, Convex lens has positive power values. Its opposite, Concave lens which is also called Diverging lens has negative focal length and as a result they have negative power value.
As per above, in case of Convex lens, P = +1/f & in case of Concave lens, D = -1/f.
6.4 Object distance is denoted by ‘u’ & image distance is denoted by ‘v’. The relationship among ‘u’, ‘v’ & ‘f’ is called lens formula.
The lens formula = 1/v – 1/u = 1/f
Exercise (Page: 197 – 198)
Q-1: The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to –
(a) presbyopia (b) accommodation (c) near-sightedness (d) far-sightedness
Ans: (b) accommodation.
Due to accommodation, the human eye can focus objects at different distances by adjusting the focal length of the eye lens.
Q-2: The human eye forms an image of an object at its –
(a) cornea (b) iris (c) pupil (d) retina
Ans: (d) retina
Retina is the layer of nerve cells lining the back wall inside the eye. This layer senses light and sends signals to the brain so that one can see.
Q-3: The least distance of distinct vision for a young adult with normal vision is about –
(a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m
Ans: (c) 25 cm
25 cm is the least distance of distinct vision for a young adult with normal vision.
Q-4: The change in focal length of an eye lens is caused by the action of the
(a) pupil (b) retina (c) ciliary muscles (d) iris
Ans: (c) ciliary muscles
The action of the ciliary muscles changes the focal length of an eye lens.
Q-5: A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Ans: If the power of a lens = ‘P’ & focal length = ‘f’ then the relationship between ‘D’ & ‘f’ is P = 1/f and ‘f’ = 1/P.
(i) Power of the lens (used for correcting distant vision) = – 5.5 D
Focal length of the lens f = 1/P = 1/-5.5 = -0.181 m
The focal length of the lens (for correcting distant vision) is – 0.181 m.
(ii) Power of the lens (used for correcting near vision) = +1.5 D
Focal length of the required lens ‘f’ = 1/P
f = 1/1.5 = +0.667 m
The focal length of the lens (for correcting near vision) is 0.667 m.
Q-6: The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Ans: The individual is suffering from myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision.
Object distance (u) = infinity = ∞
Image distance (v) = – 80 cm
Focal length = f
According to the lens formula,
A concave lens of power – 1.25 D is required by the individual to correct his defect.
Q-7: Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Ans: A person who is suffering from hypermetropia can see distinct objects clearly but he or she will face difficulty to see nearby objects. This happens because the eye lens focuses incoming divergent rays beyond the retina. This is corrected by using a convex lens. A convex lens of a suitable power converges incoming light in such a way that the image is formed on retina, as shown in the under mentioned figure.
Convex lens creates a virtual image of a nearby object at N’ in the above figure at the near point of vision (N) of the person suffering from hypermetropia.
The person will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.
Object distance, u= – 25 cm
Image distance, v= – 1 m = – 100 cm
Focal length, f
Using the lens formula,
A convex lens of power +3.0 D is required to correct the defect.
Q-8: Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: A normal eye is not able to see the objects placed closer than 25 cm clearly because the ciliary muscles of the eyes are unable to contract beyond a certain limit.
Q-9: What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: An image is formed on retina even on increasing the distance of an object from the eye. The eye lens becomes thinner and its focal length increases as the object is moved away from the eye.
Q-10: Why do stars twinkle?
Ans: The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index as the physical conditions of the earth’s atmosphere are not stationary.
Q-11: Explain why the planets do not twinkle?
Ans: Unlike stars, planets do not twinkle. Stars are so distant that they appear as pinpoints of light in the night sky even when they are viewed through a telescope. Since, all the light is coming from a single point, its path is highly susceptible to atmospheric interference, means their light is easily diffracted.
Q-12: Why does the Sun appear reddish early in the morning?
Ans: In early morning, the sun is in the horizon white light coming from the sun has to travel more distance in the atmosphere before reaching the observer. During this, the scattering of all coloured lights except the light corresponding to red colour takes place and as a result, only the red coloured light reaches the observer. Therefore, the sun appears reddish at sunrise and sunset.
Q-13: Why does the sky appear dark instead of blue to an astronaut?
Ans: The sky appears dark instead of blue to an astronaut, as scattering of light does not take place outside the earth’s atmosphere.
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