Page 64 - Physics - XII

between two uprights, the height of the needle O is adjusted so that its tip and the pole of the mirror lie in
the same horizontal line. Then, the index needle is held in such a way that its one end touches the tip of the
needle O and the other end touches the pole P or centre of the mirror.
If the distance x is measured by considering the uprights positions on the optical bench and y is the distance

between tip of the needle and centre of mirror measured by using index needle, then diff erence x-y gives
the index correction.

Principle/Theory
The mirror formula gives the relation between object distance u, the image distance v, and the focal length

f of a concave mirror, as
1 1 1
f u v
uv
or f
uv

Procedure

1. Obtain the rough focal length of the given concave mirror as follows: Mount the concave mirror in the
mirror holder and go out in the open space and face the mirror towards the distant object like tree or
building. Obtain the bright and clear image of the tree or the building on a white paper or white painted
wall. Now, measure the distance between the mirror and the image (white paper or wall) with the help
of the metre scale. This gives the rough focal length of the mirror.
2. Clamp the holder with mirror in the fi xed upright keeping mirror face towards other end of the optical
bench and the surface of the mirror perpendicular to the length of the optical bench. Place this upright

near zero end of the optical bench and two other uprights near the other end. Make mirror surface
vertical using levelling screws of the optical bench in such a way that the principal axis of the mirror
must be horizontal and parallel to the optical bench. Now, note the position of the index mark on the
base of mirror upright.
3. Mount the two needles on the two uprights and adjust their heights in such a way so that the tips of the
two needles and the pole, i.e. the centre of the mirror lie in the same straight line parallel to the length

of the optical bench. Mark O to the thin object needle and I to the thick image needle. Now, determine
the index corrections for u and v for both of the needles.
4. Move the object needle upright and clamp M
it at a distance nearly 1.5 times the obtained Image 1
rough focal length of the mirror. Since
object is situated between focal point F I C O F
and radius of curvature C, hence a real and I′ P
inverted image of object needle will be

formed beyond C as shown in Fig. 1.4. See Image Object needle
needle
this inverted image in the mirror by closing v u M
one eye and keeping the other eye open 2
along the optical bench at the height of the Fig. 1.4: Ray diagram for the focal length of

the concave mirror
pole of the mirror.
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