Refraction is one of the important phenomenons that help us to see the things around us. Light bends while travelling from one medium to another as its velocity differs from one medium to another. The speed of light in optically rarer medium is larger compared to that in optically denser medium. Hence, while travelling from one medium to another, light bends and this is called refraction.
Light ray passing from rarer to denser medium bends towards the normal. This makes the angle of incidence (angle between the incident ray and the normal at the point of incidence) larger than that of the angle of refraction (angle between the normal and the refracted ray). The incident ray, the normal and the refracted ray, all lie in a plane. If the light ray retraces its path while travelling from denser to rarer, the angle of incidence is lesser than that of the refraction. This is the principle of reversibility.
The extent to which a light ray bends depends on the refrangibility of the ray with respect to the medium. The ratio of velocity of light in vacuum to that in a medium, which is the absolute refractive index (m) of the medium, is the measure of the ability of light to get bend in the given medium. Measuring speed of light is difficult. Hence, Snell’s law helps to determine the refractive index. According to Snell’s law, .
When a light ray, incident at an angle, passes through a glass slab, the emergent ray is shifted laterally. The lateral shift depends on the thickness and refractive index of the glass slab.
When a light ray bends from denser to rarer medium, it bends away from the normal. If the angle of incidence gradually increases, the angle of refraction too increases. At a particular angle of incidence in the denser medium, the refracted ray emerges along the surface. That particular angle is the critical angle. If the angle of incidence is greater than the critical angle, the ray undergoes total internal reflection. It is due to this phenomenon we observe mirages in deserts.
he bottom of a water glass appears to rise upwards when viewed normally. This is due to the vertical shift of the bottom of the glass, which takes place because of refraction.