Now-a-days the spherical mirrors have a large number scientific and practical use. A few uses are given below:
- Doctors use concave mirrors for examination of ear, nose, throat and eyes.
- Concave mirrors with a parabolic shape are used in search light to throw an intense beam of light to a large distance.
- Some people use a concave mirror for shaving because when a man stands between the principal focus and pole of a concave mirror, he sees an enlarged, erect and virtual image of his face. This is the reason why a concave mirror of large focal length is used for shaving.
- Concave mirrors are used to throw light on the slides of microscopes so that the slides can be viewed more clearly.
- Now-a-days America and other developed countries use giant concave mirrors in their huge telescopes.
- Convex mirrors are used in motorcycles and automobiles which enable the driver to see the automobiles coming behind him.
Laws of refraction
The refraction of light takes place according to the following two laws:
- The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane (fig. 14.6).
- When a ray of light passes from one particular medium to another, the ratio of the sine of the angle of incidence (i) to the sine of
It is called snell’s law. A ray of light entering the second medium perpendicularly through the surface of separation shows no change of direction. Explain why?
Refractive index
As explained above that when light passes from one medium to another, then the constant = 
is called the refractive index of the second medium with respect to the first medium.
The refractive index of a medium can also be calculated by dividing the speed of light in vacuum by the speed of light in that medium. As the speed of light in vacuum is almost equal to the speed of light in air, therefore, we use the speed of light in air instead of speed in vacuum, while 
Refractive index of glass with respect to air =
Generally, the refractive index of transparent substances is calculated with respect to air as the speed of light is different in different substances; therefore, different substances have different ability to refract light.
