Page 62 - Physics - XII

SECTION - B

Ray Optics and Optical
4 Ray Optics and Optical
Instruments
Instruments

EXPERIMENT - 1

Aim
To fi nd the value of v for diff erent values of u in case of a concave mirror and to fi nd the focal length.

Apparatus and Materials Required

Optical bench with three uprights, concave mirror, one mirror holder, two needles, index needle, and a
metre scale

Terms and Definitions

A spherical mirror, whose refl ecting surface is curved inwards, that is, faces towards the centre of
the sphere, is called a concave mirror.
M

F Principal focus
Principal axis O
C

M′
f
Fig. 1.1: Concave mirror

1. Pole: The geometrical centre of the mirror is called pole of the mirror.

2. Centre of Curvature: It is the centre of that sphere of which the spherical mirror forms a part. It is
denoted by C.
3. Radius of Curvature: It is the distance between the pole and the centre of curvature of the mirror.
4. Principal Axis: It is the straight line passing through the centre of curvature and pole of the mirror.
5. Principal focus: It is a point on the principal axis where the rays incident parallel to the principal axis,
focus after refl ection from the surface of the mirror. It is denoted by F.
6. Focal Length: The distance between the pole P and principal focus F of the mirror is called the
focal length of the mirror. It is denoted by ‘f ’. For the mirrors of small aperture, the relation between
R
f and R is f = .
2

Cartesian Sign Conventi on

According to Cartesian sign convention:
1. All the distances are measured from the pole of the mirror.
2. The distances measured in the same direction as the direction of the incident rays are taken as positive.

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