a) What is meant by power of a lens? Name and define its Sl unit
b) One student uses a lens of focal length +50 cm and another of -50 cm. State the nature of
each lens and find their powers.
c) Which of the two lenses will always give a virtual, erect and diminished image irrespective of
the position of the object?
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1) power of a lens is its ability to refract light rays. power is equal to the reciprocal of its power. its SI unit is dioptre (symbol D). 1D = 1/1m
2) concave lens has a negative focal length while convex lens has a positive focal length. therefore the first student used a convex lens and the second used a concave lens.
power = 1/focal length
P1 = 1/50 = 0.02 D
P2 = 1/-50 = -0.02 D
3) concave lens always gives a virtual and erect image while convex lens can also give a real inverted image.
2) concave lens has a negative focal length while convex lens has a positive focal length. therefore the first student used a convex lens and the second used a concave lens.
power = 1/focal length
P1 = 1/50 = 0.02 D
P2 = 1/-50 = -0.02 D
3) concave lens always gives a virtual and erect image while convex lens can also give a real inverted image.
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