Viva Voice Questions With Answer
Focal Length of Convex Lens By Two Pin Method
For: Science Class 11 | Physics
Q.1: What is a lens?
Ans. A lens is a portion of a transparent medium bounded by two spherical surfaces or by one
spherical surface and a plane surface.
Q.2: How many principal foci are there for a lens?
Ans. Two, one on either side of the lens.
Q.3: What is the reference point for measuring p, q and f for a lens?
Ans. The optical centre of the lens.
Q.4: What kind of images are formed by convex lens?
Ans.
| Position of Object | Position of Image | Relative size of image | Nature of image |
| 1. at infinity 2. beyond C 3. at C between C & F 5. at F 6. between F & O |
at focus between F & C at ‘C’ beyond ‘C’ at infinity behind the object |
point smaller equal larger magnified magnified |
real; inverted real; inverted real; inverted real; inverted real; inverted Virtual; erect |
Q.5: What is the magnifying power (M) of a magnifying glass?
Ans. M = (1 + d/f ), where d = 25 cm = least distance of distinct vision.
Q.6: If the power of a convex lens is 0.1 diopter; what will be its focal length?
Ans. P = 1/f, so that, f = 1/p = 1/0.1 = 10m
Q.7: Which defect is present in lenses but not in mirrors?
Ans. Chromatic aberration.
Q.8: What are uses of lenses?
Ans. Combination of lenses are used in microscopes, telescopes, projectors, binoculars, cameras, etc.
Q.9: What is lens maker’s formula?
Ans. 1/f = (n – 1) (1/R1 – 1/R2)
Where,
f = focal length of the lens.
n = refractive index of the glass used for making the lens.
R1 = Radius of curvature of one surface.
R2 = Radius of curvature of the other surface.
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