Light - Part VII

Refraction of light  
Path of a light ray is always in a straight line in a given medium. If the density of the medium changes, the path of light will bend. This bending of light ray as it travels from a denser to a rarer medium or vice-versa is known as refraction. A typical example of refraction is seen if you keep a spoon in a glass of water, the spoon will appear to be bent!

Refraction occurs because the speed of light changes in the medium. It has to be remembered that when a ray of light is refracted, its speed and intensity changes but not its frequency.

Terms used for refraction
Similar to the case of reflection, we should know certain terms used for describing refraction of light.
                             
AO is the incident ray. AO is traveling in medium 1. i is the angle of incidence.

                                                         
At O the incident ray enters medium 2. OB is the refracted ray. r is the angle of refraction.

NONā is the normal. 

Rules of refraction
Rule 1 : When a light ray travels from a rarer medium to a denser medium, the light ray bends towards the normal.

In this case i   > r.  

If you keep a pencil in a glass of water, the pencil will look bent. This is because the density of water is greater than air. The light rays bend and we have an odd picture of the pencil. 

Similarly, drop a coin in a tub of water. Take another similar coin and try and drop the second coin on top of the first one. You will notice that if you drop the second coin in a straight line with respect to the first coin, you will never be able to achieve your aim. Try and drop the second coin at an angle, you will get close to your aim.

Rule 2 :  When a light ray travels from a denser medium to a rarer medium, the light ray bends away from the normal.  

In this case i   < r.

Laws of Refraction 
1. The incident ray, the refracted ray and the normal are co-planar.
2. A ray of light traveling along the normal will not get refracted and will pass un-deviated.
3. The ratio of the sine of the angle of incidence to that of the sine of the angle of refraction is constant for a given medium. This constant is called the refractive index h. This is called the Snellās Law.

Thus Snellās Law gives

  sin i
    =  
              sin r

Note that the refractive index is a mere number and is not presented by any units. It is a ratio of a quantity and hence dimensionless.  

Total Internal Reflection  
The phenomenon of total internal reflection is seen only when a light ray goes from a denser medium to a rarer medium. In this case, as seen in Rule 2, the light ray will bend away from the normal. If the angle of incidence is increased, a time will come when the refracted ray will be 90”. If you increase the angle of incidence still further, there will not be any refraction, but reflection. This phenomena is called the total internal reflection. The angle of incidence for which the angle of refraction becomes 90”, is called the critical angle.

Thus i   =   i_critical  when      r = 90”  

A nice example of total internal reflection can be seen while swimming in a clear pool of water. Try and locate a certain object on the bank of the swimming pool when your head is below the water. Now move the angle of your eyes closer to the surface of the water : you will notice that after a certain angle, all you can see is the inside of the pool and not one thing from the outside!!

Phenomena observed due to refraction and total internal reflection of light : Given below are some of the interesting observations that occur.

The phenomenon happens like this. In the desert, the air is very hot. The air near the surface of the earth is hot and is less dense. As you go up and away from the earthās surface, the air becomes cooler and hence denser. Thus there are layers of air with higher density in the vertically upward direction. Rays of light from a shady oasis area bend away from the normal as they pass from the denser layer to the rarer layer. The angle of incidence increases for each successive layer. At a certain point, the rays of light will undergo total internal reflection. When the light reaches the eyes of a weary desert traveler, to him, the light will appear to emerge in a straight line in the backward direction!  

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