Question

State whether true or false.
A concave lens is commonly called a converging lens.
$$\begin{gathered} \text{A}\text{. True} \\ \text{B}\text{. False} \end{gathered}$$

Answer 1

- Hint: To do this question, we should know about the focusing property of a concave lens, when parallel light beams fall on it.

Complete step-by-step solution -
A converging lens is the one which converges all the parallel beams of light on the focus point of the lens whereas a diverging lens diverges the beam of light falling parallel on it.
A convex lens is a converging lens, since it focuses the parallel beams of light at the real focus. For a convex lens:

For a concave lens,

The parallel beams diverge and appear to meet at the focus.
A concave lens possesses at least one surface curved inwards. It spreads out light rays that have been refracted through it, therefore it is called a diverging lens. A concave lens is thinner at its centre than at its edges, and its this property makes the light waves diverge after passing through it. So, the concave lens is a diverging lens. After light rays (parallel) have passed through the lens, they appear to come from a point called the principal focus ( the point at which the light from a distant source that moves parallel to the axis of the lens is focused).
The image formed by a concave lens:
Virtual i.e. can not be obtained on a screen and so is not a real image.
Appears to be farther than it actually is.
Is smaller than the real scale of the object.
The properties of the image formed by a concave lens, makes it useful as a corrective lens for treating short-sightedness or myopia.
So, a concave lens diverges the beam of light coming to it.
Thus, option (B) is correct.

Note: The converging and diverging nature of a lens depends on the thickness of the lens and also on the refractive index of the lens and the surrounding medium.
A concave lens can also act as a converging lens when the surrounding medium has a refractive index more than that of the material of the lens.