Find the power of a concave lens of focal length 2 m.
The power of the concave lens is -0.5 D.
Explanation
According to the textbook, the power of a lens is defined as the reciprocal of its focal length (P = 1/f). The context clearly states that 'the power of a convex lens is positive and that of a concave lens is negative.' Since a concave lens has a negative focal length, for a focal length of 2 m, we take f = -2 m. The power is then calculated as P = 1/(-2) = -0.5 D. The unit is dioptre (D) when focal length is expressed in metres.
Solution Steps
Step 1: Identify the type of lens and its focal length. The lens is concave with focal length 2 m. For a concave lens, focal length is negative, so f = -2 m.
Step 2: Apply the formula for power of a lens. Power P = 1/f = 1/(-2) = -0.5 D.
Step 3: State the final answer. The power of the concave lens is -0.5 dioptre (D).