Refraction of light
Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another. This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.
Law of refraction
The law of refraction, which is generally known as Snell’s law, governs the behaviour of light-rays as they propagate across a sharp interface between two transparent dielectric media. Consider a light-ray incident on a plane interface between two transparent dielectric media, labelled 1 and 2, as shown in Fig. 57. The law of refraction states that the incident ray, the refracted ray, and the normal to the interface, all lie in the same plane. Furthermore,
where $theta_1$ is the angle subtended between the incident ray and the normal to the interface, and $theta_2$ is the angle subtended between the refracted ray and the normal to the interface. The quantities $n_1$ and $n_2$ are termed the refractive indices of media 1 and 2, respectively. Thus, the law of refraction predicts that a light-ray always deviates more towards the normal in the optically denser medium: i.e., the medium with the higher refractive index. Note that $n_2>n_1$ in the figure. The law of refraction also holds for non-planar interfaces, provided that the normal to the interface at any given point is understood to be the normal to the local tangent plane of the interface at that point.
Refraction by glass slab
- To understand the refraction of light through a glass slab consider the figure given below which shows the refraction of light through a rectangular glass slab.
- Here in this figure AO is the light ray travelling in air and incident on glass slab at point O.
- Now on entering the glass medium this ray bends towards the normal NN’ that is light ray AO gets refracted on entering the glass medium.
- After getting refracted this ray now travels through the glass slab and at point B it comes out of the glass slab as shown in the figure.
- Since ray OB goes from glass medium to air it again gets refracted and bends away from normal N1N’1 and goes in direction BC.
- Here AO is the incident ray and BC is the emergent ray and they both are parallel to each other and OB is the refracted ray.
- Emergent ray is parallel to incident ray because the extent of bending of the ray of light at the opposite parallel faces which are PQ (air-glass interface) and SR (glass-air interface) of the rectangular glass slab is equal and opposite.
- In the figure i is the angle of incidence, r is the angle of refraction and e is the angle of emergence.
- Angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other.
- When a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium.
The critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. Make particular note that the critical angle is an angle of incidence value. For the water-air boundary, the critical angle is 48.6-degrees. For the crown glass-water boundary, the critical angle is 61.0-degrees. The actual value of the critical angle is dependent upon the combination of materials present on each side of the boundary.
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