The Refraction of light on going from one medium to another takes place according to two laws :
First law of Refraction:
The incident Ray , the refracted ray and the normal at the point of incidence, all lie in the same plane.
Second law of Refraction:
The ratio of sine angle of incidence to the sine of angle of refraction is constant for a given pair of media.
sini/sinr=Constant
The value of the constant sini/sinr for a ray of light passing from air into a particular medium is called the refractive index of the medium
sini/ sinr = n
Since the refractive index is a ratio of two similar quantities, it has no units.
The refractive index of a medium gives an indication of the light bending ability of that medium.
Refractive index of a medium :
(i) Refractive Index in terms of speed of light
The refractive index of a medium is defined as the ratio of speed of light in vaccum to the speed of light in that medium .
Refractive index = speed of light in vacuum/speed of light in medium
n = c/v
(ii) Refractive index in terms of Wavelength :
since the frequency (v) remains unchanged when light travels from one medium to another
Refractive index (n) = c/v = lamda × f/ lamda × f = lamda vacuum/ lamda vacuum
The refractive index depends on the nature of the material of the medium and on the wavelength of the light used.
If any two media are optically exactly the same , then no bending occurs when light passes from one medium to another.
A substance having higher refractive index is optically denser than another substance having lower refractive index.
Higher the refractive index of a substance , more it will change the direction of a beam of light passing through it.
The optical density of a substance is different from it's mass density.
Principal of Reversibility of light :
The refractive index for light going from medium 1 to medium 2 is equal to the reciprocal of the refractive index for light going from medium 2 to medium 1.
Absolute Refractive index :
Definition : If medium 1 is vacuum or air then the Refractive index of medium m is considered with respect to vacuum . This is called the absolute Refractive index of the medium. It is simply Represented by nm
nm = Speed of light in air / Speed of light in the medium
nm = c/ v
Relative Refractive index :
The refractive index of medium 2 with respect to medium 1 is given by the ratio of the speed of light in medium 1 and the speed of light in medium 2.
n21 = v1/v2