Refraction of light and Optical Density
Refraction of light is a phenomenon where light bends while moving from one medium to another, right at the surface separating these two mediums.
Because of this when a pencil is dipped in a glass of water, it seems to be bent just at the surface separating the water in the glass and the air outside.
In this topic we will discuss the reason behind this event called refraction of light. While discussing this we will cover ‘optical density‘ of a medium which is acting behind this refraction of light.
Now why and how does light bend during refraction ?
Light moves along straight lines as long as it travels one medium (say air). As and when it encounters a different medium (say glass) it bends at the separating surface. And then it again moves along a straight line in the second medium.
The root cause behind this bending is that the speed of light is different in different materials.
Hence when light travels from one medium to another, either its speed rises or it falls.
Now the deviation of light from its original path depends on the change in speed of light from one medium to other.
If light speeds up it bends away from the normal to the separating surface. Similarly when it slows down it bends towards the normal to the surface.
When does light speed up ? and when slows down?
When we drive on a road without much congestion we can easily drive faster if required. But in a congested road we are bound to drive slower than normal speed.
In a similar fashion light moves slower when the medium is ‘Optically Denser’ and it moves faster in an ‘Optically Rarer’ medium.
So we can say when light moves from optically denser medium to optically rarer medium, its speed rises.
As a result it moves away from the normal and gets closer to the separating surface.
On the contrary, when light moves from optically rarer to optically denser medium, its speed falls.
As a result, it moves towards the normal.
Optical Density vs Mass Density
We know that mass density is mass per unit volume. (Read our tutorial: Density vs Relative Density)
But optical density is not same as mass density. ‘Optically denser’ doesn’t mean having higher mass density.
Kerosene oil is lighter than water and floats on water i.e. mass density of kerosene is less than that of water. But on the contrary, ‘Optical density’ of kerosene is more than that of water.
That means a beam of light faces more resistance in kerosene to pass through than it does while moving through water. So speed of light is lesser in kerosene compared to that in water.
Optically denser mediums have higher refractive index (RI).
[ RI of a medium is the ratio of speed of light in air or vacuum and speed of light in that medium. ]
Conclusion – Refraction
So to conclude when light goes from the optically rarer medium to the optically denser medium, its speed falls and it bends towards the normal.
But when it moves from the optically denser medium to the optically rarer medium, it speeds up and bends away from the normal.
Thus optical density plays the key role in Refraction.