10. LIGHT – REFLECTION AND REFRACTION
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.