Definition of Refraction of light :
The change in direction of light when it passes from one medium to another obliquely is called refraction of light . In other words , the bending of light when it goes from one medium to another obliquely is called Refraction of light.
Important terms Related to Refraction:
- The plane surface that refracts light is known as the surface of refraction.
- Point of incidence: The point on the surface of refraction , where the ray of light is incident is known as the point of incidence . "O" is the point of incidence.
- Incident Ray : The ray of light which strikes the surface of refraction at the point of incidence , is known as the incident Ray . AO is the incident Ray.
- Refracted ray : The ray which travels from the point of incidence to other medium is known as the refracted ray. OB is the refracted ray.
- Normal : Perpendicular drawn on the surface of refraction at the point of incidence is called normal. NON' is the normal on the surface PQ.
- Angle of incidence: The angle between the incident Ray and the normal at the point of incidence is known as the angle of incidence. Angle of incidence is represented by the symbol i. Angle AON is the angle of incidence.
- Angle of Refraction: The angle between the refracted ray and the normal at the point of incidence is known as the angle of refraction. It is represented by symbol r. BON' is the angle of refraction.
- In refraction of light , the angle of refraction is usually not equal to the angle of incidence.
- Refraction of light takes place at the boundary between the two media.
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.
$\frac{sini}{sinr}=Constant$
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
$\frac{sini}{sinr} = n_{21} = \frac{n_{1}}{n_{2}}$
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.
If $n_{21} >1$ , $r <i$ then Refracted ray bends towards the normal. In such case medium 2 is said to be optically denser and medium 1 is rarer medium.
If $n_{21}<1$ , $r >i$ then Refracted ray bends away from the normal. In such case medium 1 is said to be optically denser and medium 2 is rarer medium.
Refractive Index in terms of speed of light and wavelength:
$n_{21}= \frac{c}{v}= \frac{lamda_{1}}{lamda_{2}}$