A short-sighted eye has a new far point of 5.00 m and a near point of 0.25 m. St

Question

A short-sighted eye has a new far point of 5.00 m and a near point of 0.25 m. State the type of lens needed to correct this defect and calculate the power of the correcting lens. Calculate the distance from the lens to the new far point of the eye with the correcting lens in front of the eye. A long-sighted eye has the far point at infinity and a near point which is 40 cm from the eye. State the type of lens needed to correct this defect and calculate the power of the correcting lens. Calculate the distance from the lens to the new near point of the eye with the correcting lens in front of the eye. A reflecting telescope has a primary mirror of diameter 20 cm and radius of curvature 2 m. If the telescope is pointed at the moon, 3480 km in diameter and 386,000 km away, find (a) the location of the moon's image and |b) the size of the moon's image, (c) Describe the moon's image. Is it real or virtual, erect or inverted, enlarged or reduced, normal or perverted? An object of height 10 cm is placed 2 m in front of a converging lens of focal length 20 cm. The light passes through that lens and then through a diverging lens of focal length 5 cm. (a) What is the distance between the two lenses if the final image is virtual and 25 cm from the diverging lens? (b) What is the height of the final image? A microscope 30 cm long is constructed from two lenses each of focal length 1 cm. (a) What is the magnifying power (angular magnification) of the microscope? (b) What would be the apparent size of a flee which is 1 mm in actual length? (c) If the angular view through the eyepiece is 45degree, could you see the whole flee in the microscope?

Explanation / Answer

1) Far point is 5m instead of infinity,

so he is suffering from myopia

Concave (diverging lens) is required

For correction, this concave lens will form the image at 5m of infinity object. So his far point becomes infinity

by lens formula,

1/infinity + 1/(-5) = 1/f

f = -5 m

Power = -0.2

new far point is at - infinity

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