Manual

6
Rotation of a Plane Mirror: Aim a single ray at the mirror and note the incident ray, mirror position
and reflected ray. Rotate the mirror slightly around the incident point and reflected ray. Measure the
angle of rotation of the mirror (∠M) and that of the reflected ray (∠R). Repeat this for different angles.
• Derive a relation between ∠M and ∠R.
The above method is used in various instruments to amplify small changes.
Concave Mirror - Focus, Center of Curvature:
• Aim a set of parallel rays at the center of the mirror such that they are parallel to the mirror’s axis
of symmetry. Note the point where they converge - this is the FOCUS. The distance of the focus
from the center of the mirror is called the FOCAL LENGTH (f). The center of curvature lies at a
distance of 2f from the mirror along this line. The distance of 2f is the radius of curvature, ‘r’ of
the mirror.
• Place a concave lens (of short focal length) in front of the slits to produce diverging rays. Shift the
mirror until the reflected rays retrace their paths. The CENTER OF CURVATURE is the point
where the incident rays appear to diverge from and the RADIUS OF CURVATURE is the distance
of the point from the mirror.
• If, in the above experiment, you are unable to obtain a position where the reflected rays retrace
themselves, use
rvuf
2111
=+=
where, f = focal length of the mirror
u = distance between the mirror and the point from where incident rays appear to diverge
v = distance between the mirror and the point where reflected rays meet
(All distances are measured along the axis of symmetry)
Concave Mirror - Caustic Curve:
SPHERICAL ABERRATION is the phenomenon in which parallel rays far away from the center of
the mirror do not focus onto the same point as those near the center, giving a distorted image.
To study aberration, allow four parallel rays to fall on the mirror and mark the points where two
adjacent rays meet. Shift the light-box to obtain another set of parallel rays, ensuring that the new set is
parallel to the old one and do the same. Join these points free-hand to obtain a curve called the
CAUSTIC CURVE.
• What causes the caustic curve?
X
C F
R
e
f
lections From a
Spherical Concave
Mirror
Fig. 2