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Class 10 Science Chapter 11 The Human Eye and the Colourful World
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The Human Eye and the Colourful World
Chapter – 11
GENERAL SCIENCE
Textual Questions and Answers:
Page – 190
Q.1. What is the meant by power of accommodation of the eye?
Ans: The power of accommodation is the ability of an eye to focus near and far objects clearly and making image on the retina by adjusting its focal length.
Q.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: Concave Lens.
Q.3. What is the far point and near point of the human eye with normal vision?
Ans: The far point of human eye with normal vision is at infinity and the near point is 25 cm distance from the eye.
Q.4. A student has difficulty reading the black board while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Ans: Since the child cannot see the distant objects (like blackboard writing ) clearly, he is suffering from the defect of vision (or defect of eye ) called ‘myopia’ or ‘short – sightedness’. Myopia can be corrected by using spectacles containing concave lenses of suitable power.
EXERCISES
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.
Q.2. The human eye forms the image of an object at its-
(a) Cornea.
(b) Iris.
(d) Retina.
Ans: (d) Retina.
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.
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.
Q.5. A person needs a lens of power -5.5 diopters for correcting his distant vision. For correcting his near di vision he needs a lens What is od ji the focal length of he lens required for correcting
(i) Distant vision and
(ii) Near vision?
Ans: (i) Given that,
p = – 5.5 D
f = ?
We have,
p = 1/f
⇒ f = 1/p
= 1/-5.5D
= 1/-5.5m
= -0.18m
(ii) Given that,
p = +1.5D
f = ?
We have,
p = 1/f
⇒ f = 1/p
= 1/+1.5D
= 1/+1.5m
= +0.666m
= +0.67m.
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 corrects the problem?
Ans: Given that,
Object distance u = ∞ (infinity)
Image distance v = – 80 cm.
focal length f = ?
We have,
1/v – 1/u = 1/f
⇒ – 1/80 – 1/∞ = 1/f
⇒ – 1/80 – 1/1/0 = 1/f
⇒ – 1/80 – 0 = 1/f
⇒ 1/f = – 1/80
⇒ f = – 80
∴ f = – 80cm.
Thus, the focal length of the required concave lens is 80cm.
Now, Power (P) = 1/f
= 1/-80cm
= – 1/0.8cm
= – 1/0.8D
= – 1.25 D
Q.7. Make a diagram to show how hypermetropia is corrected the near point of a hypermetropic eye is 1m. 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: Diagram:
2nd part:
Given that,
Object distance
u = – 25 cm / For convex lens.
Image distance
v = – 1m
= – 100 cm
Focal length
f = ?
We have,
1/v – 1/u = 1/f
⇒ 1/f = 1/-100 = 1/-25
⇒ 1/f = 1/-100 + 1/25
⇒ 1/f = – 1+4/100
⇒ 1/f = 3/100
∴ f = 100/3
= 33.3cm.
Now , Power
P = 1/f
= 1/+33.3cm.
= 1/0.33m
= + 100/33D
= + 3.0D
Q.8. Why is a normal eye not able to see clearly the objects explain placed closer than 25 cm?
Ans: Because the focal length of Normal eye is 25cm.
When any object placed closer than 25 cm then the image is FORMED behind the retina. Then object seem blurred.
Q.9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: Since the size of eyes cannot increase or decrease, the image distance remains constant. When we increase the distance of an object from the eye, the image distance in the eye does not change. The increase in the object distance is compensated by the change in the focal length of the eye lens. The focal length of the eyes changes in such a way that the image is always formed at the retina of the eye.
Q.10. Why do stars twinkle?
Ans: The stars seem to twinkle in the night sky due to the effects of the Earth’s atmosphere. When starlight enters the atmosphere, it is affected by winds in the atmosphere and areas with different temperatures and densities. This causes the light from the star to twinkle when seen from the ground.
Explanation:
- The twinkling of stars is due to atmospheric refraction of star-light. The refraction of light caused by the earth’s atmosphere having air layers of varying optical density is called atmospheric refraction.
- The physical conditions of the atmosphere keep on changing continuously due to which density of air in different layers of atmosphere also keeps on changing.
- As a result of this, the refractive index of the various layers of atmosphere also keeps on changing continuously. So, light coming from stars suffers multiple refractions and the amount of starlight reaching the eye also keeps changing and so, due to fluctuation of perceived brightness of the star, they appear like they are twinkling.
Q.11. Explain why the planets do not twinkle.
Ans: The planets are much closer to the earth. A planet can be considered as a collection of large number of point-sized sources of light. So 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. On the other hand stars twinkle because stars are point-sized sources of energy therefore the continuously changing atmosphere cause atmospheric refraction which cause variation in light.
Q.12. Why does the Sun appear reddish early in the morning?
Ans: At sunrise, the rays of light coming from the Sun need to travel long distances in the Earth’s atmosphere before they reach our eyes. On this journey, shorter wavelengths of light are scattered and only longer wavelengths reach our eyes. Blue has a short wavelength and red has a long wavelength, so red reaches the eye after atmospheric scattering. Therefore, the sun looks reddish in the early morning.
Q.13. Why does the sky appear dark instead of blue to an astronaut?
Ans: To an astronaut, the sky looks dark and black instead of blue because there is no atmosphere containing air in the outer space to scatter sunlight. So, there is no scattered light to reach our eyes in outer space, therefore the sky looks dark and black there. Since there is no scattering of blue component of white sunlight which ca reach the eyes of an astronaut in outer space, therefore the sky appears dark to the astronaut instead of blue.
Multiple Choice Questions:
Q.1. The human eye forms the image of an object at its:
(a) Cornea.
(b) Iris.
(c) Pupil.
(d) Retina.
Ans: (d) Retina.
Q.2. 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.
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 cm
Ans: (c) 25 cm
Q.4. Refraction of light in the eye occurs at:
(a) The lens only.
(b) The cornea only.
(c) Both the cornea and lens.
(d) The pupil.
Ans: (c) Both the cornea and lens.
Q.5. 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.
Q.6. A person cannot see distant objects clearly. His vision can be corrected by using the spectacles containing:
(a) Concave lenses.
(b) Plane lenses.
(c) Contact lenses.
(d) Convex lenses.
Ans: (a) Concave lenses.
Q.7. A person finds difficulty in seeing nearby objects clearly. His vision can be corrected by using spectacles containing.
(a) Converging lenses.
(b) Diverging lenses.
(c) Prismatic lenses.
(d) Chromatic lenses.
Ans: (a) Converging lenses.
Q.8. The animal which does not have eyes that 100 side ways is:
(a) Horse.
(b) Chicken.
(c) Lion.
(d) Fish.
Ans: (c) Lion.
Q.9. With both eyes open, a person’s field of view is about:
(a) 90°
(b) 150°
(c) 180°
(d) 360°
Ans: (c) 180°
Q.10. A beam of white light is shone onto a glass prism. The light cannot be:
(a) Deviated.
(b) dispersed.
(c) Focused.
(d) Refracted.
Ans: (c) Focused.
Q.11. The colour of white light which suffers the maximum bending on passing through a glass prism is:
(a) Yellow.
(b) Orange.
(c) Red.
(d) Violet.
Ans: (d) Violet.
Q.12. Which of the following colour of white light is least deviated by the prism?
(a) Green.
(b) Violet.
(c) Indigo.
(d) Yellow.
Ans: (d) Yellow.
Q.13. The splitting up of white light into seven colours on passing through a glass prism is called:
(a) Refraction.
(b) Deflection.
(c) Dispersion.
(d) Scattering.
Ans: (c) Dispersion.
Q.14. Which of the following colour of white light has the least wavelength?
(a) Red.
(b) Orange.
(c) Violet.
(d) Blue.
Ans: (c) Violet.
Q.15. The twinling of stars in due to atmospheric.
(a) Reflection of light.
(b) Dispersion of light.
(c) Interference of light.
(d) Refraction of light.
Ans: (d) Refraction of light.
Q.16. The atmosphere refraction of light causes the twinkling of:
(a) Planets only.
(b) Stars only.
(c) Planets and stars.
(d) Stars and satellites.
Ans: (b) Stars only.
Q.17. The stars appear higher in the than they actually are due to:
(a) Diffraction of light.
(b) Scattering of light.
(c) Refraction of light.
(d) Reflection of light.
Ans: (c) Refraction of light.
Q.18. The stars twinkle but the planets do not twinkle at night because.
(a) The stars are small but the planets are larte.
(b) The stars are very large but planets are small.
(c) The stars are much nearer but planets are far off.
(d) The stars are far off but planets are nearer the earth.
Ans: (d) The stars are far off but planets are nearer the earth.
Q.19. As light from a far off stars comes down towards the earth:
(a) It bends away from the normal.
(b) It bends towards the normal.
(c) It does not bend at all.
(d) It is reflected back.
Ans: (b) It bends towards the normal.
Q.20. We can se the sun before the actual sunrise by about:
(a) 5 minutes.
(b) 2 minutes.
(c) 2 hours.
(d) 20 minutes.
Ans: (b) 2 minutes.
Q.21. Due to atmospheric refraction of sunlight the time from sunrise sunset is lengthened by about:
(a) 6 minutes.
(b) 2 minutes.
(c) 4 minutes.
(d) 5 minutes.
Ans: (c) 4 minutes.
Q.22. The day is longer on the earth by about 4 minutes because
(a) The earth is round in shape.
(b) The earth rotates on its axis.
(c) The earth revolves round the sun.
(d) The earth has atmosphere.
Ans: The earth has atmosphere.
Q.23. The blue colour of sky is due to:
(a) Refraction of light.
(b) Dispersion of light.
(c) Diffraction of light.
(d) Scattering of light.
Ans: (d) Scattering of light.
Q.24. The red colour of sun at the time of sunrise and sunset is because:
(a) Red colour is least scattered.
(b) Blue colour is least scattered.
(c) Red colour is most scattered.
(d) Blue colour is most scattered.
Ans: (d) Blue colour is most scattered.
Q.25. Which of the following is not caused by the atmospheric refraction of light?
(a) Twinkle of stars at night.
(b) Sun appearing higher in the sky than it actually is
(c) Sun becoming visible two minutes before actual sunrise.
(d) Sun appearing red at sunset.
Ans: (d) Sun appearing red at sunset.
Q.26. The sky appears blue because some of the blue component of sunlight is scattered by:
(a) Gas molecules present in air.
(b) Dust particles prescut in air.
(c) Water droplets suspended in air.
(d) Soot particles present in air.
Ans: (a) Gas molecules present in air.
Q.27. Sunset kis red because at that time the light coming from the sun has to travel.
(a) Lesser thickness of earth’s atmosphere.
(b) Greater thickness of earth’s atmosphere.
(c) Varying thickness of earth’s atmosphere.
(d) Along the horizon.
Ans: (b) Greater thickness of earth’s atmosphere.
Q.28. A beam of white light falls on a glass prism. The colour of light which undergoes the least bending on passing through the glass prism is:
(a) Violet.
(b) Red.
(c) Green.
(d) Blue.
Ans: (b) Red.
Q.29. The colour of white light which is deviated the maximum on passing through the glass prism is:
(a) Blue.
(b) Indigo.
(c) Red.
(d) Orange.
Ans: (b) Indigo.
Q.30. Which of the following coloured light has the least speed in glass prism?
(a) Violet.
(b) Yellow.
(c) Red.
(d) Green.
Ans: (a) Violet.
Q.31. The colored light having the maximum speed in glass prism is:
(a) Blue.
(b) Green.
(c) Violet.
(d) Yellow.
Ans: (d) Yellow.
Q.32. Out of the following, the colour of light having the maximum wave length is:
(a) Violet.
(b) Indigo.
(c) Greene.
(d) Orange.
Ans: (d) Orange.
Q.33. Having two eyes gives a person:
(a) Deeper field of view.
(b) Colour field of view.
(c) Rear field of view.
(d) Wider field of view.
Ans: (d) Wider field of view.
Q.34. The animals of prey have:
(a) Two eyes at the front.
(b) Two eyes at the back.
(c) Two eyes on the sides.
(d) One eye at the front and one on the side.
Ans: (c) Two eyes on the sides.
Q.35. The animals called predators have:
(a) Both the eyes on the sides.
(b) One eye on the side and one at the front.
(c) One eye on the front and one at the back.
(d) Both the eyes at the front.
Ans: (d) Both the eyes at the front.
Q.36. The defect of vision which cannot be corrected by using spectacles is:
(a) Myopia.
(b) Presbyopia.
(c) Cataract.
(d) Hypermetropia.
Ans: (c) Cataract.
Q.37. A person cannot see the distant object clearly. He is suffering from the defect of vision called:
(a) Cataract.
(b) Hypermetropia.
(c) Myopia.
(d) Presbyopia.
Ans: (c) Myopia.
Q.38. Though a woman can see the distant objects clearly, she cannot sea the nearby object clearly. She is suffering from the defect of vison called:
(a) Long-sight.
(b) Short-sight.
(c) Hind-sight.
(d) Mid-sight.
Ans: (a) Long-sight.
Q.39. A young man has to hold a book at arm’s length to be able to read it clearly. The defect of vision is:
(a) Astigmatism.
(b) Myopia.
(c) Presbyopia.
(d) Hypermetropia.
Ans: (d) Hypermetropia.
Q.40. After testing the eyes of a child, the optician has prescribed the following lenses for his spectacles Left eye: +2.25 D Right eye: +2.25 D
The child is suffering from the defect of vision called:
(a) Short-sightedness.
(b) Long-sightedness.
(c) Cataract.
(d) Presbyopia.
Ans: (b) Long-sightedness.
Q.41. A person got his eyes tested the opticiav’s prescription for his spectacles reads:
Left eye: – 3.00 D Right eye: –3.50 D
The person is having a defect of vision called
(a) Presbyopia.
(b) Myopia.
(c) Astigmatism.
(d) Hypermetropia.
Ans: (b) Myopia.
Q.42. A student sitting on the last bench in the class cannot read the writing on the black board clearly but he can read the book lying on his desk clearly. Which of the following statement is correct about the student?
(a) The near point of his eyes has receded away.
(b) The near point of his eyes has come closer then.
(c) The far point of his eyes has receded away.
(d) The far point of his eyes has come closer to him.
Ans: (d) The far point of his eyes has come closer to him.
Q.43. A man driving a car can read a distant road sign clearly but finds difficulty in reading the odometer on the dashboard of the car. Which of the following statement is correct about this man?
(a) The near point of his has receded away.
(b) The war point of his eyes has come closer to him.
(c) The far point of his eyes has receded away.
(d) The far point of his eyes has come closer to him.
Ans: (a) The near point of his has receded away.
Q.44. The defect of vision in which the eye lans of a person gets progressively cloudy resulting in blurred vision is called:
(a) Myopia.
(b) Presbyopia.
(c) Colourblindness.
(d) Cataract.
Ans: (d) Cataract.
Q.45. To focus the image of a nearby object on the retina of an eye:
(a) the distant between eye and lens and retina is increased.
(b) The distance between eye-lens and retina is decreased.
(c) The thickness of eye-lens is decreased.
(d) The thickness of eye-lens is increased.
Ans: (d) The thickness of eye-lens is increased.
Q.46. The term ‘accommodation’ as applied to the eye, refers to its ability to:
(a) Control the light intensity falling on the retina.
(b) Erect the inverted image formed on the retina.
(c) Vary the focal length of the lens.
(d) Very the distance between the lens and retina.
Ans: (c) Vary the focal length of the lens.
Q.47. Which of the following controls the amount of light entering the eye?
(a) Ciliary muscles.
(b) Lens.
(c) Iris.
(d) Cornea.
Ans: (c) Iris.
Q.48. The human eye possesses the power of accommodation. This is the power to:
(a) Alter the diameter of the pupil as the intensity of light changes.
(b) Distinguish between lights of different colours.
(c) Focus on objects at different distances.
(d) Decide which of the two objects is closer.
Ans: (c) Focus on objects at different distances.
Q.49. How does the eye change in order to focus an near or distant objects?
(a) The lens moves in or out.
(b) The ratines moves in or act.
(c) The lens becomes thicker or thinner.
(d) The pupil gets larger or smaller.
Ans: (c) The lens becomes thicker or thinner.
Q.50. Which of the following changes occur when you walkout of bright sunshine in to a poorly lit room?
(a) The pupil becomes larger.
(b) The lens becomes thicker.
(c) The ciliary muscle relaxes.
(d) The pupil becomes smaller.
Ans: (a) The pupil becomes larger.
Q.51. The size of the pupil of the eye is adjusted by:
(a) Cornea.
(b) Ciliary muscles.
(c) Optic nerve.
(d) Iris.
Ans: (d) Iris.
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