Chapter 11 — The Human Eye and the Colourful World
Welcome to HSLC Guru, your trusted companion for ASSEB Class 10 Science. This chapter explores the marvellous biological optical instrument — the human eye — and the beautiful optical phenomena occurring in nature. From understanding how we see, to the defects of vision and their correction, to the splendid colours of a rainbow and the reddening of the Sun at sunset, this chapter ties together everything you have learnt about light. Read carefully, work through the questions, and you will master both the textbook problems and any conceptual question that comes in the ASSEB HSLC examination.
Chapter Summary
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 eyeball is approximately spherical with a diameter of about 2.3 cm. Light enters the eye through a thin transparent membrane called the cornea, where most of the refraction takes place. Behind the cornea lies the iris, a coloured diaphragm that controls the size of the pupil — the opening through which light enters. The pupil regulates and adjusts the amount of light entering the eye. The crystalline lens, a flexible double convex lens, focuses light onto the retina, which is a delicate light-sensitive membrane containing rod and cone cells. The retina generates electrical signals that travel through the optic nerve to the brain, where the image is interpreted.
The eye lens forms a real and inverted image of an object on the retina. The power of accommodation is the ability of the eye lens to adjust its focal length so as to focus objects at different distances clearly on the retina. This adjustment is brought about by the ciliary muscles. When the muscles relax, the lens becomes thin and its focal length increases — distant objects are clearly seen. When the muscles contract, the lens becomes thicker and its focal length decreases — nearby objects are clearly seen. The far point of a normal eye is at infinity and the near point (least distance of distinct vision) is about 25 cm.
Common defects of vision include myopia, hypermetropia, presbyopia and astigmatism. Myopia (short-sightedness) is when a person can see nearby objects clearly but not distant ones; it is corrected using a concave lens. Hypermetropia (long-sightedness) is when a person can see distant objects clearly but not nearby ones; it is corrected using a convex lens. Presbyopia arises in old age due to weakening of ciliary muscles and loss of flexibility of the lens; it is corrected using bifocal lenses. Astigmatism is caused by an irregularly shaped cornea and is corrected using cylindrical lenses. Refraction through a glass prism bends light through an angle called the angle of deviation. Dispersion of white light into its seven constituent colours (VIBGYOR) occurs because different colours refract by different amounts. A rainbow is a natural spectrum produced by dispersion, refraction and total internal reflection of sunlight by water droplets in the atmosphere.
Atmospheric refraction is responsible for the twinkling of stars (light from stars passes through air layers of varying refractive indices), advanced sunrise and delayed sunset (the Sun is visible 2 minutes before actual sunrise and 2 minutes after actual sunset). Scattering of light by molecules and fine particles explains many phenomena. The Tyndall effect is the scattering of light by colloidal particles. The blue colour of the sky arises because the molecules of the atmosphere scatter blue light (shorter wavelength) more strongly than red light. The reddening of the Sun at sunrise and sunset occurs because most of the blue light is scattered away during the long path through the atmosphere, leaving mostly red light to reach our eyes.
Textbook Question Answers
1-Mark Questions
Q1. What is the function of the iris in the human eye?
Answer: The iris is a dark muscular diaphragm that controls the size of the pupil, thereby regulating the amount of light entering the eye.
Q2. What is the least distance of distinct vision for a normal human eye?
Answer: The least distance of distinct vision for a normal human eye is about 25 cm.
Q3. Name the part of the eye on which the image of an object is formed.
Answer: The image is formed on the retina, which is a light-sensitive membrane at the back of the eye.
Q4. What is the far point of a normal human eye?
Answer: The far point of a normal human eye is at infinity.
Q5. Which colour of light has the longest wavelength in the visible spectrum?
Answer: Red light has the longest wavelength in the visible spectrum.
Q6. What is the angle of deviation in a prism?
Answer: The angle between the incident ray (extended) and the emergent ray is called the angle of deviation.
Q7. Which colour of light is scattered the most by atmospheric particles?
Answer: Blue light (shorter wavelength) is scattered the most.
Q8. What kind of lens is used to correct myopia?
Answer: A concave (diverging) lens of suitable focal length is used to correct myopia.
Q9. Who first demonstrated the dispersion of white light by a prism?
Answer: Sir Isaac Newton first demonstrated the dispersion of white light using a glass prism.
Q10. Why does the sky appear blue on a clear day?
Answer: The sky appears blue because air molecules scatter blue light from sunlight more strongly than red light.
2-3 Mark Questions
Q1. What is meant by the power of accommodation of the eye?
Answer: The ability of the eye lens to adjust its focal length so as to clearly focus objects situated at different distances on the retina is called the power of accommodation. This adjustment is achieved by the action of the ciliary muscles. When viewing nearby objects, the muscles contract, making the lens thicker and decreasing its focal length. When viewing distant objects, the muscles relax, making the lens thinner and increasing its focal length.
Q2. Why do we have two eyes for vision and not just one?
Answer: Having two eyes provides several advantages: (i) it gives a wider field of view (about 180 degrees) compared to one eye (about 150 degrees); (ii) it enables three-dimensional (stereoscopic) vision and allows us to judge distance and depth accurately; (iii) the two slightly different images formed in the two eyes are combined by the brain to produce a richer perception of the world.
Q3. Explain why stars twinkle but planets do not.
Answer: Stars are very far away from the Earth and act as point sources of light. As light from a star passes through the Earth’s atmosphere, it undergoes refraction continuously due to constantly changing refractive indices of different layers of air. This makes the apparent position of the star fluctuate slightly and the amount of light entering the eye varies — hence stars appear to twinkle. Planets, being much closer, act as extended sources (a collection of many point sources). The variations of light from the individual points average out, and the planets do not appear to twinkle.
Q4. Why does the Sun appear reddish at sunrise and sunset?
Answer: At sunrise and sunset, the Sun is near the horizon and its light has to travel through a much thicker layer of the atmosphere before reaching our eyes. Most of the shorter wavelength blue and violet light is scattered away by the air molecules. Only the longer wavelength red and orange light, which is scattered the least, reaches our eyes. As a result, the Sun appears reddish.
Q5. What is dispersion of light? Why does it occur?
Answer: The splitting of white light into its seven constituent colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a transparent medium like a glass prism is called dispersion of light. It occurs because different colours of light have different wavelengths and the refractive index of the medium is different for different wavelengths — violet light bends the most while red light bends the least, hence the colours separate.
Q6. What is the Tyndall effect? Give two examples.
Answer: The phenomenon of scattering of light by colloidal particles, which makes the path of a beam of light visible, is called the Tyndall effect. Examples: (i) When a fine beam of sunlight enters a smoke-filled room through a small hole, the beam becomes visible as the dust particles scatter the light. (ii) When sunlight passes through a canopy of dense forest, the beam becomes visible due to scattering by tiny water droplets and dust in the air.
5-6 Mark Questions
Q1. What is myopia? State its causes and explain how it is corrected with the help of a ray diagram description.
Answer: Myopia (Short-sightedness) is a defect of vision in which a person can see nearby objects clearly but cannot see distant objects clearly. In a myopic eye, the image of a distant object is formed in front of the retina instead of on it.
Causes: (i) Excessive curvature (or thickening) of the eye lens, which decreases its focal length. (ii) Elongation of the eyeball so that the retina is too far from the lens.
Correction: Myopia is corrected by using spectacles fitted with a concave (diverging) lens of suitable focal length. The concave lens diverges the parallel rays coming from a distant object so that they appear to come from the far point of the defective eye. The eye lens then forms a clear image of the object on the retina. The focal length of the corrective lens is equal to the distance of the far point from the eye.
Q2. Define hypermetropia. What are its causes? How can it be corrected?
Answer: Hypermetropia (Long-sightedness or Far-sightedness) is a defect of vision in which a person can see distant objects clearly but cannot see nearby objects clearly. The near point of a hypermetropic eye is farther than the normal near point of 25 cm. In this defect, the image of a nearby object is formed behind the retina.
Causes: (i) The focal length of the eye lens has become too long (lens becomes too flat or its converging power decreases). (ii) The eyeball has become too short (shortening of the eyeball).
Correction: Hypermetropia is corrected by using spectacles fitted with a convex (converging) lens of suitable focal length. The convex lens provides the additional convergence required so that light from an object at 25 cm appears to come from the near point of the hypermetropic eye, and the eye lens forms a clear image on the retina.
Q3. What is presbyopia? What are bifocal lenses and why are they used?
Answer: Presbyopia is a defect of vision that arises in old age. It is caused by the gradual weakening of the ciliary muscles and loss of flexibility of the eye lens, which reduces the power of accommodation. As a result, the near point gradually recedes and the elderly person finds it difficult to read or see nearby objects clearly. Sometimes a person suffers from both myopia and hypermetropia simultaneously. Such people require bifocal lenses — spectacles in which the upper portion contains a concave lens (to correct myopia for distant vision) and the lower portion contains a convex lens (to correct hypermetropia for reading and near work). Modern multifocal contact lenses can also correct presbyopia.
Q4. Describe the formation of a rainbow in the sky.
Answer: A rainbow is a natural spectrum that appears in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets present in the atmosphere. The formation of a rainbow involves three steps:
(i) Refraction and dispersion: Sunlight entering each tiny water droplet is refracted at the air-water interface. As different colours have different refractive indices, the white light is dispersed into its constituent colours.
(ii) Total internal reflection: The dispersed colours undergo total internal reflection at the back surface of the water droplet.
(iii) Refraction again: When the colours emerge from the droplet, they are refracted again. The seven colours come out at different angles. The observer, with the Sun behind, sees red on the outer side and violet on the inner side of the rainbow arc. A rainbow is always formed in a direction opposite to that of the Sun.
Q5. Explain the phenomenon of advanced sunrise and delayed sunset due to atmospheric refraction.
Answer: The Sun is visible to us about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset because of atmospheric refraction. The atmosphere consists of air layers of progressively decreasing density (and refractive index) as we go upward. When light from the Sun passes through these layers, it bends continuously toward the normal as it enters denser layers. As a result, the apparent position of the Sun appears slightly higher than its actual position. So even when the Sun is geometrically below the horizon, its light reaches us due to refraction. This produces apparent advance of sunrise and apparent delay of sunset — increasing the length of the day by about 4 minutes. The same phenomenon causes the Sun to appear oval (flattened) at the horizon, since light from the lower edge of the Sun is refracted more than light from the upper edge.
Additional Practice — MCQs
Q1. The change in focal length of an eye lens to focus the image of objects at varying distances is done by the action of the:
(a) Pupil (b) Retina (c) Ciliary muscles (d) Iris
Answer: (c) Ciliary muscles.
Q2. A person needs a lens of power –5.5 D for correction of her vision. She is suffering from:
(a) Hypermetropia (b) Myopia (c) Presbyopia (d) Astigmatism
Answer: (b) Myopia.
Q3. The human eye forms the image of an object at its:
(a) Cornea (b) Iris (c) Pupil (d) Retina
Answer: (d) Retina.
Q4. Twinkling of stars is due to:
(a) Dispersion of light (b) Atmospheric refraction (c) Reflection (d) Total internal reflection
Answer: (b) Atmospheric refraction.
Q5. The colour of light that bends the least while passing through a prism is:
(a) Violet (b) Blue (c) Yellow (d) Red
Answer: (d) Red.
Q6. The light-sensitive cells of the retina that respond to colour are:
(a) Rods (b) Cones (c) Pupils (d) Optic nerves
Answer: (b) Cones.
Q7. A bifocal lens is used to correct:
(a) Myopia only (b) Hypermetropia only (c) Both myopia and hypermetropia (d) Astigmatism
Answer: (c) Both myopia and hypermetropia.
Q8. The blue colour of the sky is due to:
(a) Reflection (b) Refraction (c) Scattering (d) Dispersion
Answer: (c) Scattering of light by air molecules.
Q9. The least distance of distinct vision for a normal adult is:
(a) 15 cm (b) 20 cm (c) 25 cm (d) 30 cm
Answer: (c) 25 cm.
Q10. A rainbow is formed due to:
(a) Reflection only (b) Refraction only (c) Dispersion, refraction and total internal reflection (d) Scattering
Answer: (c) Dispersion, refraction and total internal reflection.
Fill in the Blanks
Q1. The amount of light entering the eye is regulated by the ____________.
Answer: pupil (controlled by the iris).
Q2. The defect of vision in which a person cannot see nearby objects clearly is called ____________.
Answer: hypermetropia.
Q3. The splitting of white light into its constituent colours is called ____________.
Answer: dispersion of light.
Q4. The scattering of light by colloidal particles is known as the ____________ effect.
Answer: Tyndall.
Q5. The far point of a normal human eye is at ____________.
Answer: infinity.
True or False
Q1. The crystalline lens of the eye is a double concave lens.
Answer: False. It is a double convex (converging) lens.
Q2. Myopia is corrected using a convex lens.
Answer: False. Myopia is corrected using a concave lens.
Q3. Red light is scattered the least by atmospheric particles.
Answer: True.
Q4. The image formed on the retina of a normal eye is virtual and erect.
Answer: False. The image is real and inverted.
Q5. Atmospheric refraction makes the apparent position of the Sun higher than its actual position near the horizon.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Cornea | The thin, transparent membrane that forms the front part of the eye and where most refraction occurs. |
| Iris | The coloured muscular diaphragm that controls the size of the pupil. |
| Pupil | The small opening in the iris through which light enters the eye. |
| Crystalline Lens | The flexible double convex lens that focuses light on the retina. |
| Retina | The light-sensitive layer at the back of the eye that contains rods and cones. |
| Ciliary Muscles | Muscles that change the shape (focal length) of the eye lens. |
| Power of Accommodation | Ability of the eye lens to adjust its focal length for clear vision at different distances. |
| Far Point | The farthest point up to which the eye can see clearly (infinity for a normal eye). |
| Near Point | The closest point at which the eye can see distinctly (about 25 cm for a normal adult). |
| Myopia | Short-sightedness; corrected with a concave lens. |
| Hypermetropia | Long-sightedness; corrected with a convex lens. |
| Presbyopia | Age-related loss of accommodation; corrected with bifocal lenses. |
| Astigmatism | Defect due to irregular cornea; corrected using cylindrical lenses. |
| Dispersion | Splitting of white light into its seven constituent colours by a prism. |
| Spectrum | The band of seven colours (VIBGYOR) obtained by dispersion of white light. |
| Rainbow | A natural spectrum formed in the sky by water droplets after rain. |
| Atmospheric Refraction | Refraction of light caused by the Earth’s atmosphere having varying refractive indices. |
| Scattering of Light | The redirection of light in many directions by particles or molecules in the medium. |
| Tyndall Effect | Scattering of light by colloidal particles, making the path of light visible. |
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