If a lens can converge the sun rays at a point 20 cm. away from its optical centre, the power of this lens is — (a) + 2D
If a lens can converge the sun rays at a point 20 cm. away from its optical centre, the power of this lens is —
(a) + 2D
(b) - 2D
(c) + 5D
(d) - 51D
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