**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/r

_{1}– 1/r_{2}) , where r_{1}and r_{2}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 m*_{1}and m_{2}are the magnifications in the two postions what is the relation between these?
Ans: m

_{1}x m_{2}= 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 = (D

^{2}– d^{2})/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 L

_{1}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 L_{2}of the lens is noted. The distance between the positions L_{1}and L_{2}is measured as d. The focal length of the lens is calculated from the equation,
f = (D

^{2}— d^{2})/4D.
The experiment is repeated for different values of D and the
average value of f is calculated.

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