Physics Topics can help us understand the behavior of the natural world around us.
What Causes The Refraction of Light
We know that light travels in a straight line path. This is true as long as light rays are travelling in the same medium (or same substance) having the same density throughout. If, however, the light rays are made to go from one medium to another, the light rays change their direction (or bend) at the boundary between the two media.
For example, when a ray of light travelling in ‘air’ goes obliquely into another medium ‘glass’, it changes the direction (or bends) on entering the glass block (see the Figure). The change in direction of ray of light (or bending of ray of light) occurs again when it goes out from ‘glass’ into ‘air’ (see Figure).
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. The refraction of light takes place when light rays enter from air into glass; or from glass into air. The refraction of light takes place when light enters from air into water ; or from water into air. And the refraction of light also takes place when light enters from water into glass; or from glass into water.
The optical instruments like camera, microscope, and telescope work on the refraction of light through glass lenses. We will now understand the refraction of light more clearly with the help of a diagram.
Consider a rectangular glass slab PQRS shown in below Figure. Now, here we have two different media, one is air and the other is glass (‘media’ is the plural of ‘medium’). Please note that glass is optically denser medium as compared to air (we will give the meaning of optically denser medium after a while). Now, a ray of light AO travelling in air is incident (or falls) obliquely on the glass slab at point O (see below Figure).
Since the glass slab is transparent, most of the incident light passes into the glass slab. Since glass is an optically denser medium than air, so when the ray of light AO passes from air into glass, its direction changes, it goes along the line OB inside the glass slab and we say that the light ray has been refracted (or bent). This is because AO and OB are not in the same straight line. Please note that the refraction (or bending) of light takes place at the boundary between the two media. For example, in below figure, the refraction of light (or bending of light) takes place at point O at the boundary of the two media : air and glass. We will now define the angle of incidence and the angle of refraction.
In above figure, for the light passing from air into glass, AO is the incident ray and OB is the refracted ray. Let us draw a normal NON’ at the point of incidence O. Now, the angle between incident ray and normal (at the point of incidence) is called the angle of incidence. In Figure, the angle AON is the angle of incidence. The angle of incidence is denoted by the letter i. The angle between the refracted ray and the normal (at the point of incidence) is called the angle of refraction. In above figure, the angle N’OB is the angle of refraction. The angle of refraction is denoted by the letter r.
Please note that though in the reflection of light, the angle of reflection is always equal to the angle of incidence, but in the refraction of light, the angle of refraction is usually not equal to the angle of incidence. The angle of refraction is either smaller than the angle of incidence or greater than the angle of incidence. For example, in Figure, the angle of refraction (r) is not equal to the angle of incidence (i). In this case the angle of refraction (r) is smaller than the angle of incidence (i). We will give the reasons for this later on in this chapter. At the moment we will discuss why the refraction of light takes place on going from one medium to another.
The speed of light is different in different media (or substances). For example, the speed of light in air is 3 × 108 m/s whereas that in glass is 2 × 108 m/s. It is clear from these values that the speed of light is more in air but less in glass. In other words, light travels faster in air but slower in glass. The refraction of light is due to the change in the speed of light on going from one medium to another. Thus, when light goes from one medium to another, its speed changes. And this change in speed of light causes the refraction of light.
The refraction of light on going from air to glass takes place because the speed of light is different in air and glass : being more in air and lesser in glass. So, when light enters from air into glass, its speed changes (it gets reduced). And this change in speed of light in going from air to glass causes the refraction of light (or bending of light). Please note that greater the difference in the speeds of light in the two media, greater will be the amount of refraction (or bending) of light.
Why a Change in Speed of Light Causes Refraction of Light (or Bending of Light)
The refraction of light or change in direction of light on going from one medium to another can be explained by using the wave theory of light. A beam of light is made up of tiny waves. When a beam of light consisting of light waves and travelling in a certain medium falls obliquely (at an angle) on the boundary of another medium, then one part of the light waves enters into the other medium first and its speed changes first but the rest of waves enter the other medium a little later and hence its speed changes a little later.
The fact that the speed of light waves on one side of a beam of light changes a little before the change in speed of light waves on its other side, causes a change in the direction of light. And this change in direction of light is called refraction of light or bending of light. This will become more clear from the following example in which a beam of light is entering from air into a glass slab obliquely and then emerging into air from its other side.
Light waves travel faster in air but slower in glass. Now, when a beam of light consisting of light waves and travelling in air falls on a glass-slab obliquely then the part of light waves on the left side of the beam of light reaches the glass slab first (at point A) and slows down first on entering the glass slab (see Figure). The rest of light waves on the right side of the beam of light are still in air and have to travel more distance in air before reaching the glass slab (at point B) and hence slow down a little later on entering the glass slab (see Figure). Since the speed of left side of the beam of light is reduced a little before its
right side, therefore, the direction of light changes (or bending of light occurs) on entering the glass slab (see Figure 3 ). Please note that a decrease in speed of light waves on going from air into glass causes bending of light towards left side (towards the normal) (see above Figure).
When the beam of light travelling in glass slab comes out into air obliquely (at an angle), then the part of light waves on the left side of beam of light emerges out into air first (at point C) and speeds up first (see Figure). The part of light waves on the right side of beam of light emerges out into air (at point D) a little later and hence its speed increases a little later.
Since the speed of left side of the beam of light increases a little before its right side, therefore, the direction of light changes (or bending of light occurs) on coming out of glass slab into air (see above Figure). Please note that an increase in speed of light waves on coming out from glass into air causes bending of light towards the right side (away from the normal) (see above Figure).
When light waves move from air into glass, their speed decreases and their wavelength also decreases (they become closer). On the other hand, when light waves come out from glass into air, their speed increases and their wavelength also increases (they become farther apart). Before we discuss the refraction of light in detail, we should know the meaning of the terms ‘optically rarer medium’ and ‘optically denser medium’. This is described below.