photonsspeed-of-light
Light is made up of photons. They hit our eyes to create sensation. Suppose two points A and B are in a very long distance apart in vacuum. If we shine a very weak light from point A, practical experience dictates that from point B we would not perceive the same intensity of light (if any). If speed of light is constant, shouldn't all the photons travel from A to B and thus the intensity remain the same?
Note that they are in vacuum, photons cannot hit something and change direction.
Probably, you misunderstood the non-absolute Time interval concept. At near $c$, your eyes can't perceive that your time is dilated (and, length is contracted). You and your measurement tools won't feel any difference at near $c$. Your clocks would tick at the same rate for you like that of rest observer.
The only glitch: A rest observer won't be agree with your measured values (of time interval and length) and you won't be agree with theirs. There's nothing to understand here. It's similar to how two different observers don't agree with measured speed.
Does law of inertia has anything to do with speed of light?
Yes. Inertia is resistance to change in motion. It doesn't take much effort to get a skateboard moving, or to stop it moving. But it takes a lot of effort to get a locomotive moving, or to stop it moving. That's because the locomotive is more massive. And as Einstein said, the mass of a body is a measure of its energy content. The relationship between mass and energy is given by E=mc² where c is the speed of light. Hence inertia has something to do with the speed of light. Maybe not much, but you did say anything to do with.
Edited:
My main question is, does light travels at the same speed irrespective of from where/who/how the light is created/generated (assuming light travelling in vacuum)?
Yes, in that light doesn't overtake light. Because of the particular wave nature of light. The wave speed doesn't vary like it does for waves in the ocean. But note things like the Shapiro Delay article on Wikipedia where you can read that "the speed of a light wave depends on the strength of the gravitational potential along its path".
Meaning, the light coming from a candle or a beam from a laser gun or a sun rays or light coming from any other star takes same time to travel from Point A to Point B?
Yes. All the light travels at the same speed between A and B in vacuum. This isn't quite true in say glass or water, hence prisms and rainbows. But there are no rainbows associated with gravitational lensing.
Does energy and force what caused that light to be generated has any impact on how fast they move?
No. The speed depends on the properties of space, see Wikipedia where you can read that the speed c with which electromagnetic waves (such as light) propagate through the vacuum is related to the electric constant ε0 and the magnetic constant μ0 by the equation $c={\frac {1}{\sqrt {\varepsilon _{0}\mu _{0}}}}$.
When I think of moving of photons my basic physics knowledge which I learnt in childhood tells me that for moving anything some force has to be applied and speed and distance of the object depends on how much force is applied. I am hoping its same for the photons too.
It isn't. You can't push a photon to make it go faster or slower. That's because the photon has an E=hf wave nature. It isn't a body like a locomotive is a body. However it does offer resistance to change of motion, and you can accelerate a photon in the vector sense via Compton scattering:
Image courtesy of Rod Nave's hyperphysics
That's because it has a non-zero "inertial mass". Which is why Einstein said "if the theory corresponds to the facts, radiation conveys inertia between the emitting and absorbing bodies". Note though that inertial mass is better thought of as a measure of energy. The the meaning of the word mass has changed over the decades such that it's nowadays assumed to mean rest mass. A radiating body loses mass, and the absorbing body gains it. But the photon isn't at rest and you can't slow it down, so rest mass doesn't apply.
In that case what I wonder is the light from a match stick, lighter, torch, laser, sun or any bigger and larger star is same? They will be same unless all applies same force to the photon.
The light consists of photons of difference frequencies, and there may be differences in the polarization, but other than that I can't think of any other differences.
Best Answer
As a beam of light travels, it spreads out so that for each doubling of the distance, the intensity of the light (defined as the number of photons passing through a unit area per second) is decreased by a factor of four. This is a basic consequence of the way that light beams propagate through 3-dimensional space and has nothing to do with the speed of the beam- which does not vary with distance.
So at a point B that is distant from a light source at A, not all of the photons leaving A will hit B. The result is that fewer and fewer photons will arrive at B as the distance between A and B gets larger and larger, causing the intensity of light measured at B to decrease with increasing distance.