10. LIGHT – REFLECTION AND REFRACTION
SPHERICAL
MIRRORS
These are the mirrors
with spherical
reflecting surfaces. 2 types:
1. Concave mirror: Its reflecting surface is curved inwards (faces towards the centre
of sphere).
2. Convex mirror: Its reflecting surface
is curved outwards.
Pole (P): It is the centre of the reflecting surface of a spherical
mirror. Reflecting surface is a part of sphere.
Centre of curvature (C): It is the centre of sphere of which the
spherical mirror is a part. It is not the part of mirror and lies
outside reflecting surface.
Centre of curvature lies in front of a concave mirror. It
lies behind the convex mirror.
Radius of curvature (R): It is the radius of the sphere of which the mirror forms the
part.
The distance PC = radius
of curvature.
Principal axis: It is the imaginary straight line passing through the pole
and centre of curvature of a spherical mirror. It is normal to the
mirror at its pole.
Principal
focus (F) and Focal Length (f)
Direct the reflecting
surface of a concave mirror towards the Sun and direct the reflected light on
to a paper.
Move the paper to find a bright,
sharp spot of light on it. Wait for few minutes. The paper burns and catches
fire.
This is because the light
from the Sun is converged (concentrated) at a point of paper as a bright spot (image
of the Sun). This point is the focus of concave mirror. It produces heat
and ignites the paper.
Distance of this image
from the position of the mirror is the focal length of the mirror. This
can be represented as a ray diagram.
When several
rays parallel to the principal axis fall on a concave mirror, the
reflected rays meet/ intersect at a point on the principal axis of the mirror.
This point is called principal focus (F) of the concave
mirror.
In convex mirror,
the reflected rays appear to come from a point on the principal axis. This
point is called the principal focus of the convex mirror.
The distance between the pole
and the principal focus of a spherical mirror is called the focal
length (f).
The reflecting surface of
a spherical mirror is generally spherical and has a circular outline. The
diameter of the reflecting surface is called its aperture (the distance MN).
We consider only spherical mirrors whose aperture is much smaller than its radius
of curvature.
For spherical mirrors of small apertures, the radius of curvature is equal to twice the focal length (R = 2f). i.e., principal focus lies midway between the pole and centre of curvature.
