The first mirrors used by humans were most likely pools of dark, still water, or water collected in a primitive vessel of some sort. The requirements for making a good mirror are a surface with a very high degree of flatness (preferably but not necessarily with high reflectivity), and a surface roughness smaller than the wavelength of the light. The earliest manufactured mirrors were pieces of polished stone such as obsidian, a naturally occurring volcanic glass.
Glass was a desirable material for mirrors. Because the surface of glass is naturally smooth, it produces reflections with very little blur. In addition, glass is very hard and scratch resistant. However, glass by itself has little reflectivity, so people began coating it with metals to increase the reflectivity.
Spherical Mirror- A mirror which is made from a part of a hollow sphere.
Concave mirror- A mirror which is polished from the bulging side of a hollow sphere , such that the reflecting side is towards its hollow side.
Convex mirror- A mirror which is polished from the hollow side of the sphere, such that reflecting surface is towards its bulging side.
-The mid point of the spherical mirror is called Pole .It is denoted by the symbol P
Center of curvatureThe center of th :e sphere of which the spherical mirror is a part is called center of curvature and is denoted by symbol C.
Principal Axis- An imaginary line passing through the pole and the centre of curvature of a spherical mirror is called principal axis.
Linear Aperture- The diameter of a spherical mirror is called linear aperture.
Principal Focus-It is a point on the principal axis where a beam of light parallel to the principal axis , after reflection either actually meets or appear to meet. It is denoted by the letter F.
Focal length - The linear distance between the pole and the principal focus is called focal length. It is denoted by the letter f.
Radius of curvature-The linear distance between the pole and centre of curvature is called radius of curvature.It is denoted by the letter R.