(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D
1 Answers
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Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
We can see any object, when light reflected by that object reaches our eyes. But in the dark room, no light is reflected by the object so we are unable...
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Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm.
Hint — P = 1/f where f is in metre. Its unit is Dioptre. Lens is convex in the first case and concave in the second case. Power is equal...
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Find the focal length of a lens of power -2.0 D. What type of lens is this?
Given Power P =- 2.0D If f is the focal length of lens, then P =1/f gives f=1/P=1/-2.0 m=-50cm Negative sign shows that lens is concave. Thus focal length oflens is 50 cm and it...
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A doctor has prescribed a corrective lens of power + 1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Given P=+ 1.5D As P=1/f (in metre) , Focal length, f =1/P=1/1.5=0.667m=66.7cm As power of prescribed lens is positive, the lens is converging (or convex) lens.
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Give the SI unit of power of lens. State whether the power of a converging lens is positive or negative.
The SI unit of power of a lens is dioptre. The power of a converging lens is positive as f is +ve.
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Which defect of the eye can be corrected by using a cylindrical lens?
Astigmatisrn.
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A person needs a lens of power – 2.5 dioptres for correcting his vision. Name the defect of vision he is suffering from.
He is suffering from myopia Concave lens should be used to correct this defect of vision. f = 1/P = 1/(–2.5)m = – 4m
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A person A can clearly see objects between 25 cm and 200 cm. Which of the following
(b) 18 cm to 200 cm
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A man wearing glasses of focal length +1 m cannot clearly see beyond 1 m (a) if he is farsighted
(d) in each of these cases.
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Astigmatism is corrected using (a) cylindrical lens
(a) Cylindrical lens is used to rectify eye defect astigmatism
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