Get summaries, questions, answers, solutions, notes, extras, PDF and guides for Chapter 10 Human Eye and Colorful World: Class 10 Science textbook, which is part of the syllabus for students studying under SEBA (Assam Board), NBSE (Nagaland Board), TBSE (Tripura Board), CBSE (Central Board), MBOSE (Meghalaya Board), BSEM (Manipur Board), WBBSE (West Bengal Board), and all other boards following the NCERT books. These solutions, however, should only be treated as references and can be modified/changed.
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Summary
The human eye allows us to see the world. It uses light to form images of objects around us. The eye works much like a camera. A camera has a lens to focus light and create a picture on film or a sensor. Similarly, our eye has a lens. Light first enters through the cornea, which is the clear, curved front surface of the eye. The iris, the colored part of the eye, controls the size of the pupil. The pupil is the small dark opening in the center of the iris that lets light pass through. The lens is located behind the pupil and focuses this light onto the retina. The retina is a light-sensitive layer at the back of the eye, acting like the camera’s film. Special cells in the retina, called light-sensitive cells, detect the light and send signals through the optic nerve to the brain. The brain then interprets these signals as the images we see. The eyeball is roughly spherical and about 2.3 cm in diameter.
Our eyes have a remarkable ability to adjust their focus to see objects clearly, whether they are near or far away. This ability is called accommodation. Muscles called ciliary muscles change the shape of the eye lens. When these muscles are relaxed, the lens becomes thinner, allowing us to see distant objects clearly. When the muscles contract, the lens becomes thicker, which helps us focus on nearby objects. For a young adult with normal vision, the closest distance at which objects can be seen clearly without strain is about 25 cm. This is known as the near point. The farthest point the eye can see clearly is called the far point, which is infinity for a normal eye. Sometimes, especially as people get older, the eye lens can become cloudy. This condition is called a cataract, and it can cause blurry vision or loss of sight. Cataracts can often be treated with surgery.
Sometimes, the eye cannot focus light correctly on the retina, leading to vision defects. Myopia, also known as near-sightedness, is a condition where a person can see nearby objects clearly, but distant objects appear blurry. This happens because the image is formed in front of the retina. It can be corrected by using a concave lens. Hypermetropia, or far-sightedness, is when a person can see distant objects clearly, but has difficulty seeing nearby objects. In this case, the image is formed behind the retina. This defect can be corrected using a convex lens. Presbyopia is an age-related condition where the eye gradually loses its power of accommodation, making it difficult to focus on near objects. This occurs because the ciliary muscles weaken and the eye lens becomes less flexible. Bifocal lenses, which combine both concave and convex parts, are often used to correct presbyopia. It is also possible for people to donate their eyes after death, which can give the gift of sight to someone who is blind.
Light behaves in interesting ways when it interacts with different materials. When a beam of white light passes through a triangular piece of glass called a prism, it bends. More than that, the white light splits into a band of seven colors: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (often remembered by the acronym VIBGYOR). This splitting of white light into its component colors is called dispersion. The band of colors produced is called a spectrum. A rainbow is a beautiful natural example of dispersion. It is formed when sunlight passes through tiny raindrops suspended in the atmosphere after a rain shower. These raindrops act like tiny prisms, dispersing the sunlight into its spectrum of colors.
The Earth’s atmosphere can also bend light. This phenomenon is known as atmospheric refraction. It is the reason why stars appear to twinkle. Stars are very far away, so they appear as point sources of light. As starlight enters the Earth’s atmosphere, it passes through layers of air that have different temperatures and densities. These layers are constantly moving, causing the light from the star to bend randomly as it travels towards our eyes. This makes the star appear to flicker or twinkle. Planets, on the other hand, do not usually twinkle. This is because they are much closer to Earth and appear as extended sources of light (like tiny discs rather than points). The light from different parts of the planet also bends, but these effects average out, so the twinkling is not noticeable. Atmospheric refraction also causes the Sun to appear to rise about two minutes before it actually crosses the horizon and to set about two minutes after it has actually gone below the horizon.
Light can also be scattered by very small particles in its path. The Tyndall effect is the scattering of a beam of light as it passes through a colloid (a mixture where tiny particles are suspended). This scattering makes the path of the light beam visible. The blue color of the clear sky is also due to the scattering of sunlight. The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. They are more effective at scattering light of shorter wavelengths (like blue and violet) than light of longer wavelengths (like red). When sunlight passes through the atmosphere, the blue light is scattered in all directions. This scattered blue light enters our eyes, making the sky appear blue. Danger signal lights are often red. This is because red light has a longer wavelength and is scattered the least by fog, smoke, or atmospheric particles. Therefore, red light can be seen from a greater distance, making it suitable for signals. Astronauts in space, high above the Earth’s atmosphere, see the sky as dark instead of blue because there are no atmospheric particles to scatter the sunlight.
Textbook solutions
Intext Questions and Answers I
1. What is meant by power of accommodation of the eye?
Answer: The power of accommodation of the eye is the ability of the eye to focus on both near and distant objects, by adjusting its focal length.
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?
Answer: A person with a myopic eye cannot see distant objects distinctly. This defect can be corrected by using a concave lens of suitable power. Therefore, a concave lens should be used to restore proper vision for the person who cannot see objects beyond 1.2 m distinctly.
3. What is the far point and near point of the human eye with normal vision?
Answer: For a human eye with normal vision, the near point, which is the smallest distance at which the eye can see objects clearly without strain, is about 25 cm. The far point, which is the farthest point upto which the eye can see objects clearly, is infinity for a normal eye.
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?
Answer: The student is likely suffering from myopia, also known as near-sightedness, as a person with myopia cannot see distant objects distinctly. This defect can be corrected by using a concave lens of suitable power.
Exercise Questions and Answers
1. The human eye can focus on 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.
Answer: The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to (b) accommodation.
2. The human eye forms the image of an object at its
(a) cornea.
(b) iris.
(c) pupil.
(d) retina.
Answer: The human eye forms the image of an object at its (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.
Answer: The least distance of distinct vision for a young adult with normal vision is about (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.
Answer: The change in focal length of an eye lens is caused by the action of the (c) ciliary muscles.
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?
Answer: For correcting distant vision (myopia), a concave lens is used, and for correcting near vision (hypermetropia), a convex lens is used. The document explains that myopia is corrected by using a concave lens of suitable power, and hypermetropia is corrected by using a convex lens of suitable power. The specific focal lengths for a lens of power -5.5 dioptres for distant vision and a lens of power +1.5 dioptre for near vision are not calculated in the document.
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?
Answer: The nature of the lens required to correct the problem of myopia is a concave lens. The document states that myopia “can be corrected by using a concave lens of suitable power.” The specific power of the lens required for a myopic person whose far point is 80 cm in front of the eye is not calculated in the document.
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.
Answer: Given,
u = -25 cm
v = -1 m = -100 cm
Putting these values in the lens formula,
1/v – 1/u = 1/f
⇒ 1/f = 1/-100 – 1/-25
⇒ 1/f = -1/100 + 1/25 = 3/100
⇒ f = 100/3 cm
We know, power of lens can be calculated as, P = 1/f (in meters) or P = 100/f (in centimeters)
⇒ P = 100/(100/3)
⇒ P = 3 D
A convex lens of power + 3.0D is required to correct the defect.
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer: A normal eye is not able to see clearly objects placed closer than 25 cm because the focal length of the eye lens cannot be decreased below a certain minimum limit. If you try to read a printed page by holding it very close to your eyes, you may see the image being blurred or feel strain in the eye. To see an object comfortably and distinctly, you must hold it at about 25 cm from the eyes.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: When the distance of an object from the eye is increased, the crystalline lens adjusts its focal length to ensure the image is formed on the retina. The human eye forms the image of an object at its retina, so the image distance, which is the distance from the lens to the retina, remains the location where the image is focused for clear vision.
10. Why do stars twinkle?
Answer: Stars twinkle due to atmospheric refraction of starlight. As starlight enters the earth’s atmosphere, it undergoes refraction continuously before it reaches the earth, in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. Furthermore, this apparent position of the star is not stationary but keeps on changing slightly because the physical conditions of the earth’s atmosphere are not stationary. Since stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
11. Explain why the planets do not twinkle.
Answer: Planets do not twinkle because they are much closer to the earth and are thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect.
12. Why does the sky appear dark instead of blue to an astronaut?
Answer: The sky appears dark instead of blue to an astronaut because if the earth had no atmosphere, there would not have been any scattering, and then the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, such as an astronaut, as scattering is not prominent at such heights.
Extras
Additional MCQs (Knowledge Based)
1. What is the approximate diameter of the human eyeball?
A. 1.3 cm
B. 2.3 cm
C. 3.3 cm
D. 4.3 cm
Answer: B. 2.3 cm
43. Identify the phenomenon NOT primarily caused by atmospheric refraction.
A. Twinkling of stars
B. Advance sunrise
C. Blue colour of sky
D. Delayed sunset
Answer: C. Blue colour of sky
Additional MCQs (Competency Based)
1. Assertion (A): A person with hypermetropia requires a convex lens to correct their vision.
Reason (R): In a hypermetropic eye, the light rays from a nearby object are focused at a point behind the retina.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
24. Match the term related to vision in Column A with its definition or characteristic in Column B.
| Column A (Term) | Column B (Definition/Characteristic) |
| (i) Power of Accommodation | 1. The farthest point up to which the eye can see objects clearly |
| (ii) Near Point | 2. The ability of the eye lens to adjust its focal length |
| (iii) Far Point | 3. The minimum distance for distinct vision (approx. 25 cm) |
| (iv) Spectrum | 4. Band of coloured components of a light beam |
Select the correct code:
(a) (i)–2, (ii)–3, (iii)–1, (iv)–4
(b) (i)–1, (ii)–4, (iii)–2, (iv)–3
(c) (i)–2, (ii)–1, (iii)–3, (iv)–4
(d) (i)–3, (ii)–2, (iii)–4, (iv)–1
Answer: (a) (i)–2, (ii)–3, (iii)–1, (iv)–4
Additional Questions and Answers
1. What is the function of the cornea in the human eye?
Answer: Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.
23. Discuss age-related vision defects such as presbyopia and cataract, and the corrective measures available.
Answer: Presbyopia is an age-related vision defect where the power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away, and they find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Corrective measures include the use of corrective eye-glasses. Sometimes, a person may suffer from both myopia and hypermetropia along with presbyopia, and such people often require bi-focal lenses. A common type of bi-focal lenses consists of both concave and convex lenses; the lower part is a convex lens, which facilitates near vision. These days, it is also possible to correct refractive defects like presbyopia with contact lenses or through surgical interventions.
Cataract is another age-related vision defect where, sometimes, the crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract and it causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
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