Light: Reason you see through
Bending of light is a rather common phenomenon for our universe as it can be seen everywhere, from the vastness of universe where the photons released from a distance galaxy is bent by the gravity of another galaxy to a rather reachable place of our refrigerator where an ice packet bends the light passing through it. But bending of light by a galaxy and bending of light by ice blocks might sound same but they have completely different reasons and are described by completely different physics. The first mentioned phenomenon is called Gravitational Lensing. It is a phenomenon that occurs when a huge amount of mass like galaxies, star cluster or galaxy cluster create such heavy distortion in space that it magnifies and also curves the light that was emitted by distant galaxies that are hidden behind it. Einstein’s general theory of relativity described this phenomenon that is created due to the curve in spacetime fabric. It was first observed directly on 14 September, 2015. Before that we could only see the affects of gravitational lensing. It is not only the huge masses that can cause gravitational lensing. Stars like our own Sun can also warp the fabric of space creating a similar phenomenon called Gravitational Microlensing. In fact this very concept practically proved Einstein Theory of Relativity. Well than it is not only general relatively proving gravitational lensing but also gravitational lensing proving general relativity. Sometimes, gravity can bend and focus light like a lens in a magnifying glass or pair of glasses. When a star passes in front of another star, the light of the distant star becomes temporarily brighter. Gravitational Microlensing in present time is being used to spot exoplanets as well. 136 such planets have been discovered by this method.
It is not only gravity that can bend light. We have another such phenomenon what we are going to discuss and understand today. What do we call them? Refraction, I presume.
Refraction is a common phenomenon. We all have read about it. What is their to learn about it? You may ask. Well refraction has a deeper cause. One of the misconception about refraction is that, light bends when it travels from one medium to another because the speed of light slows down. Well the slowing part is true but the bending part is not. Light bends when it travels from one medium to another because of change in electric field not due to change in its speed.
Causes of both reflection and refraction is light being an electromagnetic wave. For light to refract it must travel from one medium to another. For light to travel through a medium, the medium most be transparent or somehow transparent. So, Refraction can only occur in transparent medium.
As I mentioned in my previous article, Light waves are created due to oscillation of electric and magnetic field with are mutually perpendicular to the direction of propagation of light. Electric field is the trump card for refraction as well.
1)
The perpendicular electric field being a vector has x-component and y-component. When light enters from one medium to another the x-component remains same whereas the y-component shortens or lengthens. When light enters from rarer to denser medium the y-component shortens and when it enter from denser to rarer medium it lengthens. The y component shortens due to the repulsive force exarted by charge present in a denser medium.
2)
The charges in a transparent medium are randomly arranged, that is, they are not polarized. When light enters in the surface, the electric field of the light interferes in the randomness of the charges due to which they get polarized and a counterbalancing electric field is imposed on the electric field of light. Due to this the y-component of electric field shortens.
3)
Notice how the magnitude of electric field decreases once the y-component shortens. And it shifts downwards as well. Since, propagation of light is perpendicular to electric field. The direction of light bends downwards too.
4)
This is short theoretical description of refraction of light.
Now lets view this mathematically.
For this we need to play some tricks with maxwell’s wave equation and should know some basic vector.
We are going to use the first and the third one only.
In the previous paragraphs, I mentioned that the electric field of light has x-component and y component.
Where,
The component of electric field are different in different mediums. To describe the relation between them we must look into the Boundary Conditions of electromagnetism. It describes about how the electric field and magnetic field change when they travel from one medium to another medium. Boundary Conditions are rather a vast concept and has a lot to understand.
Basically, by the methods of Boundary Conditions we come into conclusion that:
In case of light travelling from rarer to denser medium
X-component of electric field:
Electric field in denser medium = Electric field in rarer medium
which means that the horizontal component of electric field in all medium is always equal.
For Y-component of electric field:
Permittivity of denser medium * Electric field of denser medium = Permittivity of rarer medium * Electric field of rarer medium
which means that the Vertical component will be:
Electric field of denser medium = Electric field of rarer medium * Permittivity of rarer medium / Permittivity of denser medium
Since, the Permittivity of denser medium > Permittivity of rarer medium. So, the quotient of the relation is always less than 1.
Therefore, Electric field in denser medium < Electric field in rarer medium.
Since, the vertical component is less, the electric field will also be less. And since the direction of light is perpendicular to electric field, the light will bend.
Now you know, Light refracts because of its x and y components and the Permittivity of the medium. I hope this article was helpful in understanding concept of physics. I have left some important topic in this article which we will be discussing in future. Thank you.
