Question

What is the power of a concave lens?
a. Positive
b. Negative
c. Neutral
d. None

Answer 1

- Hint: Power of a lens is defined as reciprocal of focal length. For concave lenses the power is negative because the focal length is negative.

Complete step-by-step solution :

In optics, power is defined as the ability of the optical instrument to bend the rays of light falling on it. If an instrument bends the rays towards the optical axis then power is defined as positive. If an instrument bends the rays away from the optical axis then power is defined as negative. If the instrument does not bend rays then power is zero.
Power of a lens (P) is the reciprocal of its focal length (f). The S I unit of power of lens is Dioptre represented by D.
Mathematically,
$p=\dfrac{1}{f}$
We know that a concave lens diverges the rays incident on it. That’s why it is also known as a diverging lens. Hence, its power is negative.
So, option (b) is correct.

Additional Information: Power of convex lens is positive as it converges the light rays incident on it. Convex lens is also referred to as a converging lens.
For mirrors there is a slight difference in the formula of power but the meaning is the same.
For mirror, mathematically power is
$p=-\dfrac{1}{f}$
So the focal length of the concave mirror is negative. So, power for a concave mirror is positive.
Also, the focal length of the convex mirror is positive. So, power for a concave mirror is negative.

Note: Students should always read whose power is asked by the lens or mirror. The focal length of a concave surface is negative, whereas the focal length of a convex surface is positive. Therefore, for a lens, the power of a concave and convex lens is negative and positive respectively. Whereas, for a mirror, the power of concave and convex lens is positive and negative respectively.