Refraction refers to the property by which light bends when it travels from rarer to a denser or from a denser to a rarer medium. If a ray of light is travelling from a denser to a rarer medium, say from glass to air it bends away from the normal. If the ray of light travels from a rarer to a denser medium, it bends towards the normal.
The phenomenon of refraction of light is governed by laws known as Laws of Refraction. When a ray of light traverses from one medium to another, the direction and the angle by which it bends is determined by these laws. The first law of refraction states that the incident ray, the refracted ray and the normal all lie in the same plane (Walker et al, 2010)
The second law is prime factor for determining the direction of the refracted ray. This law is popularly known as Snell’s law. According the this law the direction of the refracted ray depends on the ratio of speed of light in the two mediums.
Light travels with different speeds in different mediums. Most materials have refractive indices of their own which primarily depends on the wavelength of light. The refractive index of any material is given by the ratio of the speed of light travelling of light in that medium by the speed of light travelling in vacuum. Optically dense medium are those that have a high refractive index while those materials whose refractive indices are almost near to 1 are known as optically rare media (Haynes, 2014)
When light travels from a medium of higher refractive index to a medium of lower refractive index, the path taken by the light rays is dependent on the angle of incidence. If the angle of incidence exceeds beyond a certain value the refracted ray does not travel to another medium but in fact returns back into the same medium. This phenomenon is known as total internal reflection and the angle of incidence is known as the critical angle.
Critical angle is also known as the limit angle and is defined as the angle