Why light moves with a constant speed?

 You always heard that the speed of light is constant, means it doesn't change. But why it is constant? Today in this post I will try explain it. But before that let me introduce the light.

- Light is a form of energy which travels in the form of electromagnetic wave, means it consist both electric and magnetic field both perpendicular to each other. 

- Light wave is a transverse wave. And it can travel without any medium.

- It's speed slows down in dense transparent/translucent medium and speed up in lighter medium. Fastest speed in vacuum.

- It is massless.

Let understand why the speed of light is constant. Suppose that you are in a car and your friend is standing on road the side. Your car is moving with speed 40km/hr when you look outside, you'll find everything is going backward with the same speed as you're going forward. But when you look inside the car, you'll find everything is at rest. At the same time your friend is perceiving that your car and you moving with 40km/hr and surrounding is at rest. So who is previewing right you or your friend?

Well, both of you are correct according to your reference. So, to study any motion we take a frame of reference. 

When we will see that car from different point of reference then we will find that the speed of car is different for different reference. When we see it from a truck moving in its direction with same speed (40km/hr) then we will see it as rest. When the speed of truck is 30km/hr then it will be going forward with speed of 10km/hr. And so on.

But what is the actual speed of the car? Is that 40km/hr? If we take earth as reference then it is 40, but earth is also moving. Our whole solar system is moving. Means we can never tell the actual speed of the car, because nothing is at absolute rest. 

Now let consider as wave, yes we can consider it, as it shows dual nature. So once light is emitted from a source

Will be continued...

Check out tomorrow