The human eye and the colourful world Worksheet-4
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The far point of a myopic eye is at 50 cm. Calculate the power of the lens to correct his vision.
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The far point of a myopic eye is 80 cm in front of the eye. What is the power of the lens required to enable him to see very distant objects clearly?
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The near point of a hypermetropic eye is at 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cm from the eye ?
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A person cannot see the objects distinctly, when placed at a distance less than 100 cm. What is the power of the spectacles that he should use to see clearly the objects placed at 25 cm?
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A short sighted person cannot see clearly beyond 2 m. Calculate power of the lens required to correct his eye to normal vision.
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A myopic person can see things clearly only when they lie between 10 cm and 100 cm from the eye. Which lens will enable him to see the moon clearly ?
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The distance of distinct vision of a person is 50 cm. He wants to read a book placed at 25 cm. What should be the focal length of the spectacles?
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A person wants to read a book placed at 20 cm, whereas near point of his eye is 30 cm. calculate the power of the lens required.
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The far point of a myopic person is 4 meter. Calculate the power of the lens he requires to look at the stars.
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A myopic person uses specs of power –0.5D. What is the distance of far point of his eye ?
Answer:
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Here, distance of far point x = 50 cm, power P = ?
As f = –x ∴ f = –50 cm
As
Negative sign indicates that the lens used is concave.
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Distance of far point x = 80 cm,
power, P = ?
f = –x f = –80 cm
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Here, distance of near point, x' = 75 cm
distance of book,
power P = ? & focal length f = ?
We know that
As f is positive in this case so the corrective lens is convex.
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Given : distance of near point, x' = 100 cm
distance of object, d = 25 cm.
power, P = ?, focal length, f = ?
As
As
As f is positive in this case so the corrective lens is convex.
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distance of far point, x = 2m., power P = ?
As f = –x
∴ f = –2 m = –200 cm
As
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Distance of far point, x = 100 cm.
To see the moon clearly, he has to use spects with lens of focal length,
f = –x = –100 cm
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x' = 50 cm, d = 25 cm, f = ?
From = 50 cm.
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d = 20 cm, x = 30 cm, P = ?
= 60 cm.
= 1.67 D.
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x = 4m, P = ?
f = –x = –4m
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P = –0.5 D, x = ?