CBSE Class 10 Science The Human Eye and the Colourful World Notes

The Human Eye and the Colourful World is Chapter 11 of CBSE Class 10 Science. These notes cover the parts of the human eye, working of the eye, power of accommodation, near point and far point, defects of vision (myopia, hypermetropia, presbyopia and astigmatism), dispersion of white light, rainbow formation, atmospheric refraction, twinkling of stars, scattering of light and the Tyndall effect, with the intext and exercise questions.

THE HUMAN EYE

The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colours around us.

The main parts of the human eye include:
Cornea: transparent tissue covering the front of the eye that lets light travel through.
Iris: a ring of muscles in the coloured part of the eye that controls the size of the pupil.
Pupil: an opening in the centre of the iris that changes size to control how much light enters the eye.
Sclera: the white part of the eye, made of fibrous tissue that protects the inner workings of the eye.
Lens: located directly behind the pupil, it focuses light rays onto the retina.
Retina: membrane at the back of the eye that changes light into nerve signals.
Optic nerve: a bundle of nerve fibres that carries messages from the eyes to the brain.
Macula: a small and highly sensitive part of the retina responsible for central vision, which allows a person to see shapes, colours and details clearly and sharply.
Choroid: a layer of blood vessels between the retina and sclera; it supplies blood to the retina.
Ciliary muscle: changes the shape of the lens (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts, rounding out the lens.
Aqueous humour: a water-like fluid produced by the ciliary body. It fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients. It drains back into the blood stream through the canals of Schlemm.
Vitreous humour: the space between the lens and retina, filled with the gel-like vitreous humour.

[Figure: Labelled diagram of the human eye showing aqueous humour, cornea, pupil, iris, ciliary ligament, lens, vitreous humour, ciliary muscle, retina, sclera, choroid, optic nerve and the bulge of the cornea.]

WORKING OF THE HUMAN EYE

Light enters the eye through a thin membrane called the cornea, which forms the transparent bulge on the front surface of the eyeball. The eyeball is approximately spherical, with a diameter of about 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina.

Behind the cornea is the iris, a dark muscular diaphragm that controls the size of the pupil. The pupil regulates the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina. Light-sensitive cells get activated on illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets the signals and processes the information so that we perceive objects as they are, that is, without inversion.

[Figure: Cross-section of the eye showing light entering through the cornea, pupil, lens and vitreous humour, with the image formed on the retina; the optic nerve carries signals to the optic centre of the brain.]

POWER OF ACCOMMODATION

The process by which the ciliary muscles change the focal length of the eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye.

How does an eye focus objects at varying distances?
To focus on distant objects, the ciliary muscles relax, making the eye lens thin. As a result, the focal length of the eye lens increases and we see the distant objects. To focus on nearby objects, the ciliary muscles contract, making the eye lens thick. As a result, the focal length of the eye lens decreases and we see the nearby objects. In short, it is the adjustment of the focal length of the eye lens that enables us to focus on objects at different distances.

[Figure: Accommodation (contracted muscle) and relaxed muscle diagrams showing a thick lens for a near object and a thin lens for a distant object, with rays converging on the retina.]

NEAR POINT OR LEAST DISTANCE OF DISTINCT VISION

The near point, or least distance of distinct vision, is the point nearest to the eye at which an object is visible distinctly. For a normal eye it is about 25 cm. It varies with the age of the person; for infants it is only 5 to 8 cm.

FAR POINT

The far point is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.

RANGE OF VISION

The distance between the near point and the far point is called the range of vision.

DEFECTS OF VISION

A normal eye can see objects over a wide range of distances, from 25 cm to infinity. Due to certain abnormalities, the eye may not be able to see objects over this range, and such an eye is said to be defective. The defects of vision are:
Hypermetropia or long-sightedness
Myopia or short-sightedness
Presbyopia
Astigmatism

HYPERMETROPIA

Hypermetropia is also known as far-sightedness. It is the defect of the eye due to which the eye cannot see nearby objects clearly, though it can see distant objects clearly. The near point of the person is farther away than the normal near point (25 cm). Such a person has to keep reading material much beyond 25 cm from the eye for comfortable reading. This is because light rays from a close object are focused at a point behind the retina.

This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. It can be corrected by using a convex lens of appropriate power. Eyeglasses with converging lenses provide the additional focusing power required for forming the image on the retina.

[Figure: Hypermetropic eye with the image formed behind the retina, and the correction using a convex lens that brings the image onto the retina.]

MYOPIA

Myopia is also known as near-sightedness. A myopic person cannot see distant objects clearly because the far point of the eye is less than infinity. Myopia is caused by:
the elongation of the eyeball, that is, the distance between the retina and the eye lens is increased; and
a decrease in the focal length of the eye lens.

In a myopic eye, the image of a distant object is formed in front of the retina and not on it. This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. It can be corrected by using a concave lens of suitable power, which brings the image back onto the retina.

[Figure: Myopic eye with light falling too short of the retina, and the correction using a concave lens that forces the light to fall on the retina.]

PRESBYOPIA

Presbyopia occurs at about the age of 40 years. Its main symptom is reduced near vision: difficulty in reading without glasses at 35 to 40 cm and fatigue after a short period of close work. Normally the lens is flexible enough to change its shape when focusing on close objects. Loss of its flexibility and elasticity, known as loss of the eye's adjustment mechanism, results in presbyopia. The name means "aging eye".

At a young age the lens is soft and flexible and changes shape easily. After the age of 40 the lens becomes more rigid, so it is more difficult to read at close range. Since nearly everyone develops presbyopia, if a person also has myopia, hypermetropia or astigmatism, the conditions combine.

[Figure: Three diagrams of the eye: normal eye with the image on the retina; presbyopic eye with the lens hardened and unable to change shape, so the image falls behind the retina; presbyopia corrected with eyeglasses or contact lenses.]

ASTIGMATISM

Astigmatism is an eye condition with blurred vision as its main symptom. The cornea of a person with astigmatism is not curved properly; the curve is irregular, usually one half is flatter than the other, and sometimes one area is steeper than it should be. Astigmatism may also be caused by an irregularly shaped lens behind the cornea.

Astigmatism is a type of refractive error. Light is not focused correctly on the retina, so the image is blurred. The two most common types are:
Corneal astigmatism: the cornea has an irregular shape.
Lenticular astigmatism: the lens has an irregular shape.

In astigmatism, images focus in front of and beyond the retina, so both close and distant objects appear blurred.

[Figure: Normal eye with images focused on the retina, and astigmatic eye with the cornea irregularly shaped, so images focus in front of and beyond the retina.]

INTEXT QUESTIONS (PAGE No. 190)

1. What is meant by power of accommodation of the eye?
Ans: When the ciliary muscles are relaxed, the eye lens becomes thin, the focal length increases, and distant objects are clearly visible. To see nearby objects clearly, the ciliary muscles contract, making the eye lens thicker. The focal length of the eye lens decreases and nearby objects become visible. The ability of the eye lens to adjust its focal length to view both distant and nearby objects on the retina is called the power of accommodation of the eye.

2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Ans: The person can see nearby objects clearly but cannot see objects beyond 1.2 m, because the image of such an object is formed in front of the retina. To correct this, he must use a concave lens, which brings the image back to the retina.

[Figure: Myopic eye with the image formed in front of the retina for an object beyond 1.2 m, and the same eye with a concave lens bringing the image back to the retina.]

3. What is the far point and near point of the human eye with normal vision?
Ans: The near point of the eye is the minimum distance of an object from the eye that can be seen distinctly without strain. For a normal human eye this is 25 cm. The far point is the maximum distance up to which the eye can see objects clearly. For a normal human eye it is infinity.

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 unable to see distant objects clearly, so he is suffering from myopia. This defect can be corrected by using a concave lens.

REFRACTION OF LIGHT THROUGH A PRISM

A prism is a transparent optical element that refracts light. An optical object to be defined as a prism must have at least two faces with an angle between them. A triangular glass prism has two triangular bases and three rectangular lateral surfaces, which are inclined to each other. The angle between its two lateral faces is called the angle of the prism.

In the ray diagram, PE is the incident ray, EF is the refracted ray and FS is the emergent ray. A ray of light entering from air to glass at the first surface AB bends towards the normal. At the second surface AC, the ray enters from glass to air and bends away from the normal. The shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation, denoted D.

[Figure: Triangular glass prism ABC with incident ray PE, refracted ray EF and emergent ray FS. Labels: angle of incidence i, angle of refraction r, angle of emergence e, angle of deviation D, angle of the prism A. Normal lines are drawn at E and F.]

DISPERSION OF WHITE LIGHT BY A GLASS PRISM

When a ray of light enters the prism, it bends towards the normal, because light enters from a rarer medium into a denser medium. When the light emerges from the prism, it again follows the laws of refraction. Because of the angle of the prism and the different wavelengths of the components of white light, the emergent ray is separated into different colours. A coloured band of seven colours is obtained. This phenomenon is called dispersion of white light by the prism.

[Figure: White light entering a glass prism and emerging as a band of seven colours, from red at the top to violet at the bottom, which falls on a screen.]

RAINBOW FORMATION

A rainbow is a natural spectrum that appears in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets present in the atmosphere. A rainbow is always formed in the direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, reflect it internally, and refract it again when it comes out of the raindrop. Due to dispersion and internal reflection, different colours reach the observer's eye.

[Figure: Sunlight entering a spherical water drop, refracting, reflecting internally and emerging as a spectrum; an observer sees the rainbow in the direction opposite to the Sun.]

ATMOSPHERIC REFRACTION

Atmospheric refraction is the shift in the apparent direction of a celestial object caused by the refraction of light rays as they pass through the Earth's atmosphere.

TWINKLING OF STARS

Stars emit their own light and twinkle due to atmospheric refraction. Stars are very far away, so they are considered point sources of light. When light from a star enters the Earth's atmosphere, it is refracted at different levels because of variation in air density. When the refracted starlight comes more towards us, the star appears brighter; when it comes less towards us, it appears dimmer. Therefore, the stars appear to twinkle at night.

[Figure: Apparent star position due to atmospheric refraction, with the actual star position and the higher apparent position, and the ray path bending towards the observer as refractive index increases.]

ADVANCE SUNRISE AND DELAYED SUNSET

The Sun is visible about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset because of atmospheric refraction. Actual sunrise means the actual crossing of the horizon by the Sun. The figure below shows the actual and apparent positions of the Sun with respect to the horizon. 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 atmospheric refraction.

SCATTERING OF LIGHT

In the air, part of the sunlight is scattered by small particles such as molecules, tiny water droplets and dust particles. Shorter wavelengths are scattered more. In the scattered light, short wavelengths predominate, so the sky appears blue, while direct sunlight appears somewhat yellowish, or even reddish when the Sun is very low.

TYNDALL EFFECT

The Earth's atmosphere is a mixture of minute particles, including smoke, tiny water droplets, dust particles 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 scattering of light by colloidal particles gives rise to the Tyndall effect. It is seen when a fine beam of sunlight enters a smoke-filled room through a small hole, and when sunlight passes through the canopy of a dense forest.

WHY IS THE COLOUR OF THE CLEAR SKY BLUE?

The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. They scatter light of shorter wavelengths (the blue end) much more strongly than light of longer wavelengths (the red end). Red light has a wavelength about 1.8 times that of blue light. So the blue colour is scattered more and reaches our eyes. Without an atmosphere there would be no scattering and the sky would look dark. The sky appears dark to passengers flying at very high altitudes, where scattering is not prominent.

COLOUR OF THE SUN AT SUNRISE AND SUNSET

Light from the Sun near the horizon passes through a thicker layer of air and a larger distance in the atmosphere before reaching our eyes. Light from the Sun overhead travels a shorter distance. At noon the Sun appears white, as only a little blue and violet light is scattered. Near the horizon most of the blue light and shorter wavelengths are scattered away, so the light reaching our eyes has longer wavelengths. This gives the Sun its reddish appearance.

[Figure: Sun at noon overhead with a short path through the atmosphere, and Sun near the horizon with a long path through the atmosphere.]

EXERCISE QUESTIONS (PAGE No. 197 and 198)

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. The eye lens changes its focal length to see objects at various distances, and this is the power of accommodation.

2. The human eye forms the image of an object at its (a) cornea (b) iris (c) pupil (d) retina
Ans: (d) retina.

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.

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. Their relaxation or contraction changes the curvature of the eye lens and hence its focal length.

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: (i) For distant vision, P = -5.5 D, so f = 1/P = 1/(-5.5) m = -0.181 m. The focal length of the lens for correcting distant vision is -0.181 m.
(ii) For near vision, P = +1.5 D, so f = 1/P = 1/1.5 = +0.667 m. The focal length of the lens for correcting near vision is 0.667 m.

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 person has myopia, so a concave lens is needed. For the lens to form the image of an object at infinity at the far point, the image distance is v = -80 cm and u = infinity. Using 1/v - 1/u = 1/f, f = -80 cm = -0.8 m. Power P = 1/f = -1/0.8 = -1.25 D. The lens is a concave lens of power -1.25 D.

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: Hypermetropia is corrected by a convex lens that forms a virtual image of the object placed at 25 cm, at the eye's near point of 1 m.
Object distance u = -25 cm; image distance v = -1 m = -100 cm (the PDF prints -100 m, which is a typing error; the correct value is -100 cm).
Using 1/v - 1/u = 1/f: 1/f = -1/100 + 1/25 = 3/100, so f = 100/3 cm = 0.333 m.
Power P = 1/0.333 = +3.0 D. A convex lens of power +3.0 D is required.

[Figure: Hypermetropic eye with a convex lens placed in front of it, so that a virtual image of an object at 25 cm is formed at the near point of 1 m.]

8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: The ciliary muscles of the eye cannot contract beyond a certain limit. An object placed closer than 25 cm appears blurred and causes strain in the eyes.

9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: The image distance in the eye remains constant, because the size of the eye cannot change. The increase in object distance is compensated by a change in the focal length of the eye lens, so the image is always formed on the retina.

10. Why do stars twinkle?
Ans: Stars emit their own light and twinkle due to atmospheric refraction. As they are very far away they are point sources, and light from them is refracted at different levels in the atmosphere, making the star appear to brighten and dim.

11. Explain why the planets do not twinkle?
Ans: Planets appear larger than stars because they are relatively closer to the Earth. A planet can be considered a collection of a large number of point-size sources of light. The brighter and dimmer effects of these point sources average out to zero, so the twinkling of planets is nullified.

12. Why does the Sun appear reddish early in the morning?
Ans: At sunrise the light from the Sun travels a greater distance through the atmosphere. Shorter wavelengths are scattered out, and only the longer wavelengths reach our eyes. Red has the longer wavelength, so the Sun appears reddish.

13. Why does the sky appear dark instead of blue to an astronaut?
Ans: There is no atmosphere in outer space to scatter sunlight. No scattered light reaches the astronaut's eyes, so the sky appears black.

WORKING OF HUMAN EYE

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