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.
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