# Concave Lens Viva Questions

Concave Lens Viva Questions with Answers

(i) Why is a concave lens known as a diverging lens?

Ans: When a parallel beam of light is incident on a concave lens the beam emerges as a divergent beam. Hence the lens is called a diverging lens.

(ii) What is the nature of the image formed when an object is placed in front of a concave lens?

Ans: The images, virtual, erect diminished and is formed within the focus of the lens

(iii) In the experiment to find the focal length of a concave lens by combination method the focal length of the convex lens must be smaller than that of the concave lens. Why?

Ans: So that the combination behaves as a convex lens and forms real images.

(iv) A convex lens of focal length +10 cm and a concave lens of focal length -10 cm are brought in contact. What is the focal length of the combinations?

Ans: Infinity

v) How will you distinguish between a convex lens and a concave lens?

Ans: The lens is placed close to a printed matter. If the image is magnified the lens is convex. If the image of the print is diminished the lens is concave.

vi) Can we form magnified image with a concave lens.

Ans: No. The concave lens can form only diminished images.

vii) Aim of the Concave Lens Experiment

Ans: To find the focal length of the given concave lens using an auxiliary convex lens (i) in contact and (ii) out of contact.

viii) Apparatus of the Concave Lens Experiment

Illuminated wire gauze, concave lens, short focused convex lens, screen, lens holder, etc.

ix) Theory of the Concave Lens Experiment

Ans:
(1) Combination method (Lenses in contact):

If F is the focal length of the combination of a convex lens of focal length f' and concave lens, of local length f , then,
1/F = 1/f’ + 1/f
Therefore, f = Ff’/(f’ - F)

(2) Auxiliary convex lens method (Lenses out of contact)

If u is the object distance and v is the virtual image distance from the concave lens.
f = uv/(v - u)

x) Procedure of the Concave Lens Experiment

Ans:

(1). Lenses in contact (combination method)

The focal length (f’) of the convex lens is found out by displacement method or u - v, method as described in previous blog post. The convex lens is then kept in contact with the concave lens and focal length (F) of the combination is found out by u – v method. If f is the focal length of the concave lens, then,
1/F = 1/f’ + 1/f
Therefore, f = Ff’/(f’ - F)

(2). Lenses out of contact (Auxiliary convex lens method)

The convex lens is placed in front of the illuminated wire gauze and the screen is adjusted on the other side of the lens so that a clear image of the wire gauze is obtained on the screen. The position I1 of the screen is noted. The concave lens L is then interposed between the convex lens and the screen. The image becomes blurred. The screen is then moved away from the lens until a clear image is obtained on it. The position I2 of the screen is noted. With respect to the concave lens if an object is placed at I2, a virtual image will be obtained at I1. Hence LI2 can be taken as u and LI1 as v. Then,
1/f = 1/u – 1/v
Therefore, f = uv/ (v - u)
The experiment is repeated for different positions of the lens and the mean value of f is found out.