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Studying Physics Topics can lead to exciting new discoveries and technological advancements.
What do you mean by optically denser and optically rarer mediums?
A transparent substance in which light travels is known as a medium. Air, glass, certain plastics, water, kerosene, alcohol, etc., are all examples of medium. Different media are said to have different optical densities. A medium in which the speed of light is more is known as optically rarer medium (or less dense medium). Air is an optically rarer medium as compared to glass and water. A medium in which the speed of light is less, is known as optically denser medium. Glass is an optically denser medium than air and water. Now, the speed of light in water is 2.25 × 108 m/s, which is less than that in air but more than that in glass.
So, water is optically denser medium than air but it is optically rarer than glass. Please note that the refraction of light takes place whenever it goes from an optically rarer medium to an optically denser medium; or from a denser medium to a rarer medium. The optically rarer medium and optically denser medium can also be defined on the basis of their refractive index. We will learn this after a while. Keeping the above discussion in mind, we can now write down two rules which give the direction of bending of a ray of light when it goes from one medium to another.
It has been found that:
- When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal (at the point of incidence).
- When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal (at the point of incidence).
We will now understand these rules more clearly with the help of ray-diagrams.
Case 1: Refraction of Light When it Goes From a Rarer Medium to a Denser Medium
Below Figure shows a ray of light AO going from air (a rarer medium) into glass (which is a denser medium). In this case, the incident ray AO gets refracted at point O and bends towards the normal ON’ and goes in the direction OB inside the glass slab.
Thus, when a ray of light goes from air into glass, it bends towards the normal (at the point of incidence). In this case, the angle of refraction (r) is smaller than the angle of incidence (i).
Water is also an optically denser medium than air, so when a ray of light goes from air into water, it bends towards the normal.
Thus, the refraction of light on going from air to water is similar to the refraction of light from air to glass (which has been shown in above Figure). To show the path of a ray of light going from air into water, we can use this Figure but we will have to write ‘water’ in place of ‘glass’ and in place of glass slab we will have to show water by drawing some dotted lines. Please draw the diagram to show the refraction of a ray of light from air to water yourself.
Case 2 : Refraction of Light When it Goes From a Denser Medium to a Rarer Medium
Below Figure shows a ray of light AO going from glass (a denser medium) into air (which is a rarer medium). In this case, the A incident ray AO gets refracted at point O and bends away from the normal ON’ in the direction OB. Thus, when a ray of light goes from glass into air, it bends away from the normal (at the point of incidence). In this case the angle of refraction (r) is greater than the angle of incidence (i).
Please note that water is also optically denser than air, so when a beam of light travelling in water enters into air, it bends away from the normal. We can show the refraction of light on going from water into air by using the ray-diagram given in above Figure. All that we have to do is to write ‘water’ in place of glass’ and Normal
show water by drawing some dotted lines. Please draw the diagram to show the refraction of a ray of light from water to air yourself. Please note that a parallel-sided glass slab is also called rectangular glass block.
When a ray of light goes from a rarer medium to a denser medium, its speed decreases or it slows down. On the other hand, when a ray of light goes from a denser medium to a rarer medium, then its speed increases or it speeds up. So, we can now say that a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal but when a ray of light travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.
The Case of Light Going From Air into Glass and Again into Air
We have just studied the refraction of light from air into glass (Figure), and from glass into air (Figure). We will now show these two pes of refraction in the same diagram. In other words, we will now show the complete path of a ray of light when it passes from air into glass slab and again into air. This is shown in below Figure.
A ray of light AO travelling in air is incident on a rectangular glass slab PQRS at point O. On entering the glass slab, it gets refracted along OB and bends towards the normal ON’ (see Below Figure). A second change of direction takes place when the refracted ray of light OB, travelling in glass emerges (or comes out) into air at point B. Since the ray of light OB now goes from a denser medium ‘glass’ into the rarer medium ‘air’, it bends away from the normal BN1’ and goes in the direction BC.
Please note that the incident ray AO Direction and the emergent ray BC are parallel to each other original ray (though the emergent ray has been displaced parallel to the incident ray). The incident ray AO and emergent ray BC are parallel to each other because the extent of bending, of the ray of light at points O and B on the opposite, parallel faces (PQ and SR) of the rectangular glass slab is equal and opposite. The incident ray AO Below Figure. bends towards the normal at point O whereas the refracted ray OB bends away from the normal at point B by an equal amount.
Thus, the light emerges from a parallel-sided glass slab in a direction parallel with that in which it enters the glass slab. Though the emergent ray BC is parallel to the incident ray AO, but the emergent ray has been sideways displaced (or laterally displaced) from the original path of the incident ray by a perpendicular distance CD (see Figure).
In Figure, the original path of incident light is AOD but the emergent light goes along BC, the lateral displacement between them being CD. The perpendicular distance between the original path of incident ray and the emergent ray coming out of the glass slab is called lateral displacement of the emergent ray of light. Lateral displacement depends mainly on three factors : angle of incidence, thickness of glass slab, and refractive index of glass slab. Actually, lateral displacement is directly proportional to
- angle of incidence
- thickness of glass slab, and
- refractive index of glass slab.
Higher the values of these factors, greater will be the lateral displacement. Another point to be noted is that in this case the refraction (or bending) of light takes place twice: first at point O (when light enters from air into glass), and then at point B (when light goes out from glass into air).
The angle which the emergent ray makes with the normal is called the angle of emergence. In above Figure, the angle N1BC is the angle of emergence. Since the incident ray AO and the emergent ray BC are parallel to one another, so the angle of emergence (e) is equal to the angle of incidence
(i). Please note that if a beam of light travelling in air passes into water and then emerges into air, we will get a ray-diagram similar to that shown in above Figure. Draw it yourself by writing ‘water’ in place of ‘glass’.
The Case of Light Falling Normally (or Perpendicularly) on a Glass Slab
In all our discussions so far we have assumed that the incident ray of light falls obliquely to the surface
of glass slab and bending of ray of light takes place. If the incident ray falls normally (or perpendicularly)
to the surface of a glass slab, then there is no bending of the ray of light, and it goes straight. For example, in Figure, a ray of light AO travelling in air falls on a glass slab normally (or perpendicularly) at
point O, so it does not bend on entering the glass slab or on coming out of the glass slab. It goes straight in
the direction AOBC. Since the incident ray goes along the normal to the surface, the angle of incidence in
this case is zero (O) and the angle of refraction is also zero (O).
We can explain the case of w refraction (or no bending) of light on going perpendicularly from air to glass as follows: When a beam of light consisting of light waves and travelling in air falls perpendicularly (at right angles) to the surface of glass slab, then all the parts of light waves reach the glass slab at the same time, enter the glass slab at the same time and hence get slowed down at the same time (see Figure).
Due to this no change in direction of light takes place. In other words, no refraction of light (no bending of light) takes place. Similarly, all the parts of the light waves of the beam of light travelling in glass slab come out of the glass slab into air at the same time and hence speed up at the same time. Due to this, no bending of light occurs when the light waves go out from glass slab into air (see Figure).
Please note that if a ray of Light falls normally (or perpendicularly) to the surface of water, even then there is no bending of light ray, and it goes straight through water.