Convex Lens Viva Questions with Answers


Convex Lens Viva Questions with Answers
(i) What is the minimum distance between the object and its image for a convex lens?

Ans: 46

(ii) Define focal length of a convex lens

Ans: It is the distance between optic centre and principal focus

(iii) What is lens makers formula?

Ans: 1/f = (n — 1)(1/r1 – 1/r2) , where r1 and r2 are the radii of curvature of the two faces.

(iv) What is meant by power of a lens?

Ans: It is the reciprocal of the focal length expressed in metres, p = 1/f

(v) What is the unit of power?

Ans: Dioptre

(vi) Which type of lens is used in microscopes?

Ans: Convex lenses

(vii) What is the shape of 1/u — 1 /v-graph?

Ans: The graph is straight line with equal intercepts

(viii) Are the positions of the object and image interchangable

Ans: Yes, for real images only

(ix) In the displacement method we get magnified and diminished images for two postions of the lens between the object and screen. If m1 and m2 are the magnifications in the two postions what is the relation between these?

Ans: m1 x m2 = 1

(x) Can a convex lens form virtual image?

Ans: Yes, when the object is between F and C

(xi) What is the property of the optic centre?

Ans: A ray passing through the optic centre passes without deviation

(xii) Aim of Convex Lens Experiment

Ans: To find the focal length of the given convex lens

(xiii) Apparatus of Convex Lens Experiment

Ans: Convex lens, screen, illuminated wire gauze, etc.

(xiv) Theory of Convex Lens Experiment

Ans: If u ad v are the object distance and the image distance from the convex lens, its focal length f = uv/(u + v). If D is the distance between the object and the screen, and d is the distance between the conjugate positions of the lens,

f = (D2 – d2)/4D

(xv) Procedure of Convex Lens Experiment

Ans: The focal length of the convex lens can be found out by different methods.

1. Distant object method

The convex lens is faced to a distant scenery and the position of the screen is adjusted so that a well defined image of the scenery is formed on the screen. The distance between the lens and the image on the screen gives the focal length of the lens. The experiment is repeated and the mean focal length is found out.

2. u-v method

The convex lens is placed in front of the illuminated wire gauze. A screen is adjusted on the other side of the lens so that a clear image of the object is formed on it. The distances of the object and the image from the lens are measured as u and v. The focal length is then calculated using the formula,
f = uv/(u + v)
The experiment is repeated for different values of u. Readings are taken for both magnified and diminished images.

3. u-v graph

A graph is drawn with u along the X-axis and v along the Y-axis taking a common origin and same scale for both axes. A bisector to the angle XOY is drawn which meets the graph at P. Then OA = OB =2f. Hence the focal length can be found out.

4. 1/u - 1/v graph

A graph is drawn with 1/u along the X axis and 1/v along the Y axis taking zero as origin and same scale for both axes. The graph is a straight line intercepting the axes at A and B. Then OA = OB = 1/f; from which f can be calculated.

5. Displacement method

The screen is placed at a distance more than 4f from the object, the illuminated wire gauze. The distance between the object and the screen is measured as D. The convex lens is placed nearer to the object in between the object and the screen. Its position is adjusted to get well defined enlarged image of the object on the screen. The position L1 of the lens is noted. The lens is moved towards the screen till a well defined diminished image of the object is obtained on the screen. The position L2 of the lens is noted. The distance between the positions L1 and L2 is measured as d. The focal length of the lens is calculated from the equation,
f = (D2 — d2)/4D.
The experiment is repeated for different values of D and the average value of f is calculated.

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