The person has myopia (cannot see distant objects) and hypermetropia (cannot see very near objects).
To see distant objects (infinity): The far point is 400 cm. The corrective lens must form an image of an object at infinity (u = -infinity) at 400 cm (v = -400 cm). Using 1/f = 1/v - 1/u, f = -400 cm = -4 m. Power P1 = 1/f = 1/(-4) = -0.25 D. (Concave lens)
To see objects at 25 cm: The near point is 50 cm. The corrective lens must form an image of an object at 25 cm (u = -25 cm) at 50 cm (v = -50 cm). Using 1/f = 1/v - 1/u, 1/f = 1/(-50) - 1/(-25) = -1/50 + 2/50 = 1/50. f = 50 cm = 0.5 m. Power P2 = 1/f = 1/(0.5) = +2.0 D. (Convex lens)
Therefore, he needs bifocal lenses, with a concave lens of -0.25 D for distant vision and a convex lens of +2.0 D for near vision.
More Questions on The Human Eye and the Colourful World
1Define the terms 'near point' and 'far point' for the human eye. What are their normal values for a healthy young adult?
→ 2Assertion (A): The Sun appears elliptical at sunrise and sunset. Reason (R): Atmospheric refraction causes the Sun's apparent vertical diameter to decrease more than its horizontal diameter, leading to an elliptical appearance.
→ 3Why do raindrops cause dispersion of white light to form a rainbow, but air bubbles in water do not produce a similar visible spectrum?
→ 4Is the image formed on the retina real or virtual? Is it inverted or erect?
→ 5Emergency exit signs in buildings are often green. What scientific reason supports this choice, considering human vision?
→ 6In the dispersion of white light by a prism, which colour deviates the most and which deviates the least? What is the reason for this difference in deviation?
→ 7A person needs a concave lens of power -4.5 D for correcting his distant vision. Calculate the focal length of the corrective lens and the distance of his far point.
→ 8If the Earth had no atmosphere, what would be the colour of the sky during the day?
→ 9If a beam of white light is passed through a glass slab, parallel to its base, it emerges as white light. However, if it passes through a glass prism, it disperses into its constituent colours. Explain this difference.
→ 10What is cataract, and how is it corrected?
→ 11Why is there a difference in the extent of scattering of red and blue light by the atmosphere?
→ 12A person has a near point of 100 cm and a far point of 500 cm. He wants to be able to read a book at 25 cm and see distant objects. Calculate the powers of the bifocal lenses he needs.
→ 13Why does the pupil of the eye appear black?
→ 14Two prisms, one made of crown glass and the other of flint glass, are used. For a given angle of incidence, which prism would produce a greater dispersion of white light and why?
→ 15Which type of photoreceptor cells are primarily responsible for our ability to differentiate colours? Where are they most concentrated?
→ 16Why is red light scattered the least compared to other colours in the visible spectrum?
→ 17What is meant by the 'power of accommodation' of the human eye?
→ 18A student is unable to see the words written on the blackboard clearly when sitting on the front bench, but he can read the book in his hand without any problem. Identify the eye defect and suggest the type of lens to correct it.
→ 19Explain the phenomenon of 'persistence of vision' and its application.
→ 20A person needs a concave lens of focal length 50 cm to correct his distant vision. What is the farthest distance this person can see clearly without corrective lenses?
→ 21Why do clouds generally appear white?
→ 22Why are danger signals usually red in colour?
→ 23Why do different colours of light bend at different angles when passing through a prism?
→ 24Can a person with colour blindness see any colours at all? Explain briefly.
→ 25The near point of a hypermetropic person is 75 cm from the eye. Calculate the power of the lens required to enable him to read clearly a book held at 25 cm from the eye.
→ 26A child has difficulty seeing objects far away but can read a book comfortably. What is his eye defect? What kind of lens would a doctor recommend?
→ 27Which part of the eye is donated in eye donation, and why is it important?
→ 28Why does a glass prism cause dispersion of white light, while a hollow prism or a glass slab does not?
→ 29How does the human eye change its focal length to focus on objects at different distances?
→ 30Assertion (A): Red colour is used as a danger signal. Reason (R): Red light has the longest wavelength and is scattered the least by smoke or fog.
→ 31A light ray enters a water droplet from the air. It strikes the inner surface of the water droplet and undergoes total internal reflection. Which condition must be met for this total internal reflection to occur?
→ 32Compare the function of the retina in the human eye with a part of a photographic camera.
→ 33What is presbyopia? How is it corrected?
→ 34An astronaut in space finds the sky to be dark instead of blue. Why?
→ 35A person uses a convex lens of focal length 25 cm to read a book. What is the power of this lens?
→ 36Explain why we see the Sun two minutes before the actual sunrise and two minutes after the actual sunset.
→ 37Differentiate between rod cells and cone cells in the retina.
→ 38What is the role of the ciliary muscles in the human eye?
→ 39Which part of the human eye controls the amount of light entering the eye?
→ 40Describe the formation of a rainbow.
→ 41Why is the Tyndall effect observed when light passes through a colloidal solution but not through a true solution?
→ 42Draw a labelled diagram to show the dispersion of white light by a glass prism. Write the sequence of colours observed on the screen.
→ 43Stars twinkle but planets do not. Explain this phenomenon.
→ 44Why does the Sun appear reddish at sunrise and sunset?
→ 45The sky appears blue during the day. Explain why.
→ 46A student sitting on the last bench in the class complains that he is unable to read the writing on the blackboard. What is the likely defect of vision he is suffering from? How is this defect corrected?
→ 47Why can a person with myopia not see distant objects clearly? How is this defect corrected?
→ 48A person needs a lens of power -5.5 D for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 D. Calculate the focal length of the lens required for correcting (i) distant vision and (ii) near vision.
→ 49What is hypermetropia?
→ 50Can visible light be scattered by atoms/molecules in the earth’s atmosphere?
→ 51Why do different colours deviate through different angles on passing through a prism?
→ 52What is the function of the optic nerve in the human eye?
→ 53Why are we not able to see things clearly when we come out of a dark room?
→ 54Why do we observe random wavering or flicking of the objects near a fire or on a very hot day?
→ 55State the role of the eye lenses in the human eye?
→ 56Why is a convex lens called a converging lens?
→ 57What is the role of the ciliary muscles?
→ 58State the difference in colours of the sun observed during sunrise/sunset and noon. Give explanation for each.
→ 59Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.
→ 60Why is Tyndall effect shown by colloidal particles? State four instance of observing the Tyndall effect.
→ 61What is a spectrum? How can we recombine the components of white light after a glass prism has separated them?
→ 62A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
→ 63Name the three common defects of vision. What are their causes? Name the type of lens used to correct each of them.
→ 64List two causes of hypermetropia.
→ 65Write the function of retina in human eye.
→ 66List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function?
→ 67Why does the sky appear dark instead of blue to an astronaut?
→ 68Explain why the planets do not twinkle.
→ 69Why do stars twinkle?
→ 70What happens to the image distance in the eye when we increase the distance of an object from the eye?
→ 71Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
→ 72The 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?
→ 73A 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?
→ 74The change in focal length of an eye lens is caused by the action of the (a) pupil. (b) retina. (c) ciliary muscles. (d) iris
→ 75The 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
→ 76The human eye forms the image of an object at its (a) cornea. (b) iris. (c) pupil. (d) retina.
→ 77The 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
→ 78A 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?
→ 79What is the far point and near point of the human eye with normal vision?
→ 80A 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?
→ 81What is meant by power of accommodation of the eye?
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