Question

The power of a lens is +8D. What kind of lens is this?
 (A) Convex
(B) Concave
(C) Concavo Convex
(D) Convexo concave

Answer 1

Hint Convex lenses are thin at the edges at budget at the middle whereas concave lenses are thin at the centre and budget at the sides. Power of a lens is positive for a convex lens because focal length of the convex lens is positive.

Complete step-by-step solution
Power of a lens is defined as the reciprocal of focal length in meters. Its unit is in Dioptre. Focal length of a convex lens is positive because it is a converging lens. As the power of a lens is reciprocal of the focal length, the convex lenses have positive power.

Therefore, the correct answer is A.

Note: We can find the focal length of this lens in m by doing a reciprocal of power which will give us focal length as 0.125 m or 12.5 cm. Power of the concave lens is negative as the focal length of the concave lens is negative. As power is the reciprocal of focal length the reciprocal of a negative focal length is again a negative value. Hence, a concave lens has negative power.