Viva Voice Questions With Answer
Focal Length of Concave Lens By Concave Mirror
For: Science Class 11 | Physics

Q.1: What is a concave lens?
Ans. The lens which is thinner at the centre and thicker at the edges is known as concave lens.

Q.2: What is the difference between a convex lens and a concave lens?
Ans. (a) Sides of a convex lens are bulging whereas the surface of the concave lens are deep towards the optical centre. (b) Real images of surrounding objects can be obtained by a convex lens on the screen like a wall. This cannot be done by a concave lens.

Q.3: Can you find the focal length of a concave lens without using an auxiliary apparatus?
Ans. No, an auxiliary apparatus like plane mirror, a convex lens or a concave mirror must be used to find the focal length of a concave lens because:

(a) the image formed by the concave mirror is on the same side as the object and
(b) the image formed by a concave lens is always virtual.

Q.4: What is the maximum distance of the image formed by a concave lens?
Ans. Focal length of the lens.

Q.5: What is the distance of the image formed by the concave lens from the concave mirror when the parallax is removed?
Ans. The distance is equal to the radius of curvature of the concave mirror.

Q.6: Can you get final image of the object if the distance of the concave lens and the concave mirror is greater than the radius of curvature of the mirror?
Ans. No, because the rays passing through the lens can never fall normally on the mirror and the final image cannot be formed at the object. Thus parallax can never be removed.

Q.7: How many images are formed in this experiment?
Ans. (a) virtual image of the object formed by the concave lens.
(b) the real image of the first image (virtual object) formed by concave mirror.
(c) the final image of the second image formed by the concave lens.

Q.8: Which lens is used for the correction of short-sightedness (myopia)?
Ans. Concave lens.

Q.9: How long-sightedness (hypermetropia) is correct?
Ans. By using a convex lens.

Q.10: What are other uses of concave lenses?
Ans. (a) As on eye-piece in the Gallilean telescope.
(b) In making achromatic combinations.