**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 I

_{1}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 I_{2}of the screen is noted. With respect to the concave lens if an object is placed at I_{2}, a virtual image will be obtained at I_{1}. Hence LI_{2}can be taken as u and LI_{1}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.

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