Question

Find the focal length of a lens of power −2.0D. What type of lens is this?

Answer 1

First write the formula for the power of the lens. Now, put the value of power that is given in the question in the formula for the power of the lens. If there is a negative sign then it depicts the type of lens is concave.

Complete step by step answer:
The lens given is a concave lens as it has a negative value for power.
$\text{Power,}\ \,P=\dfrac{1}{f(\text{in metres})}\ \,\ \,(f=\text{focal length})$
$\text{So,}\ \,-2.0=\dfrac{1}{f}$
$\text{And,}\ \,f=-\dfrac{1}{2.0}m$
$\text{Or}\ \,f=-\dfrac{1}{2.0}\times 100cm$
So, Focal length of lens,
f = - 50 cm

Additional Information:
The unit of power is D(diopter).
The power of a lens is the ability to converge or diverge the ray of light falling on it. The power of a lens is defined as reciprocal of the focal length of the lens. A lens of small focal length has a large power of converging or diverging a parallel beam of light. The power of the lens is 1 diopter if its focal length is 1 m.
The power of the lens is 1 diopter if its focal length is 1 m.

Application Of lens
- Convex lenses are used in cameras, astronomical telescopes, and terrestrial telescopes.
- A convex lens of a small focal length is used in a microscope to study biological specimens.
- It is used in reading glass.
- It is used to correct hypermetropia defects.

Note:
The negative sign of the power of lens acts in the equation also. The relation of power with focal length is a must. The negative sign depicts the type of lens that is, so remember the criteria for concave lens and convex lens.