Light: Reason you see it
Light is much younger than our universe. To be precise a mockingly 380000 years younger but rather a small period in the cosmic lifetime. Our universe is about 13.8 billion years old. And by saying light is younger, I didn’t mean that photons didn’t existed 13.79962 billion years ago. I mean to say that light couldn’t travel along the expansion of universe. Since it couldn’t travel, it had no significance in being light. Within minutes of Big Bang, the smaller universe created billions of protons , electrons, neutrons and their anti behavioural friends. In this scenario, the photons of energy couldn’t travel through the unspaced space. Each and every time they tried to march forward, they encountered an electron whizzing around with its charge. The electrons struck and scattered the hardworking photons.
The process continued for the first 380000 years. What happened after 380000 years? Well its simple, the temperature of the universe dropped. It dropped below 3000 kelvin. The free electrons that were torturing tiny photons were no longer free. The electrons were attracted by previously forged nuclei consisting of proton and neutrons. ( Universe created this stuff after one second of its birth ). The photons broke free from the torture of the electron and travelled through the universe in a path made up of oscillating electric and magnetic fields. These restless photons are still travelling through the universe. They did lose some of there energy though and now they are Microwave photons. They can be observed all over the sky (though not observed by our eyes). They create, now what we call Cosmic Microwave Background(CMB).
It is not only the Big Bang that traps photons. Our Sun traps the photon inside it for about 1000000 years. Yes, it takes a photon about 1000000 years to migrate from core of the sun to its surface. The reason is the same, ELECTRONS! Once the photons reach the surface they take only 500 seconds to fill our eyes.
There has always been debates and contradictory theories on the topic, What actually makes us see? The Greek Philosophers had many theory. According to Plato and Euclid, just like The Superman our eyes produces some kind of rays. These rays strikes an object just before we can see it. It was Aristotle who pointed out the falsity in this theory. He said that if this was true than we should be able to see during dark as well.
The world knows that these theories are incorrect. The reflection and refraction of light makes us see object. We see it because of reflection and we see through because of refraction.
Reflection and Refraction are the two basic properties of light. When light strikes a surface it returns back making an angle equal to the incident angle. When light passes through one medium to another, it changes its course. Yes, we all know about them or do we…..? What really causes light to bounce back or light to bend? What are the reasons of refraction and reflection?
Light waves are electromagnetic waves. They have oscillating electric and magnetic fields that are mutually perpendicular to the direction of propagation of light.
When a ray of light strikes a surface, the charges of the surface go inside the electric field of light. We all know that a charge present in an electric field experiences a force which is equal to:
F=q*E
1)
So, the negative charge(electron) of the surface starts to oscillate due to this force.
2)
Oscillating charged particles emit electromagnetic waves. ( I will write another article describing this topic ).
The electrons can oscillate in any direction and the electromagnetic waves can be emitted in every direction. Here, once again the nature makes things easy for us. The electromagnetic radiations emitted by electrons cancel each other out by destructive interference in every direction except one where there is constructive interference.
Constructive interference happens when the trough and trough, and crest and crest of two waves combine with each other to form another wave.
Destructive interference is when the trough and crest combine with each other that destroys the wave.
The constructive interference during reflection is seen in a particular direction where the angle made by the reflected ray makes angle with the normal which is equal to the angle made by the incident ray with the normal.
1)
2)
This is how light reflects by maintaining the laws of reflection.
This means that light applies force, doesn’t it? How can light apply force when it doesn’t have mass? These may be the first questions that may rise in your mind. Well light can apply force. And this property of light is being used or is going to be used in some of the of the most extraordinary projects ever thought by humans.
The property is going to be used in Breakthrough Starshot project. An idea created in the beautiful mind of Stephen Hawking. The project aims to travel to our neighbor star system within 20 years by travelling at 1/5th the speed of light. It involves a ground-based light beamer pushing ultra-light nanocrafts.
Another place where this property of light is being used is National Ignition Facility(NIF), California, USA. They use 192 laser beams with terawatts of power to conduct nuclear fusion reaction. They aim to produce pollution free electricity.
To understand why light exerts force without having mass, you need to know a little quantum mechanics. Even though light doesn’t have mass, it has momentum and anything that has momentum can apply force.
We have tons of relations and theories describing this. But lets take help from De Broglie wave equation. De Broglie in 1924 AD proposed the duality nature of electron and gave a relation describing it.
P =h/λ
where, P is momentum, h is plank constant, λ is wavelength.
This relation is applicable for all moving particles. It says that less the wavelength, more the momentum and more the momentum, more the force.
So, if light can exert force without having mass than Newton’s second law of motion is gone for. Well, that is not the case here. You see Newton’s second law of motion has a second representation. That is
F = P/t
Now you know, Light reflects because is applies force. I hope this article was helpful in understanding physics. Stay tuned for the refraction part. You are going to have a lot of fun reading that.
