Why most of physics is somehow related to light? It seems that for the past 200 years, every physicist is concerned about light. For example : Newton's particle model, Young experiment, Photo-Electrict effect and Einstein's formula, Special Relativity (constant speed of light), Bohr's atom model (using Photons to emit electrons), Double-slit experiment from the quantum aspect etc.
My question is - What's so special about light and why do we care so much about it? I'm currently at 12th grade and all I've learned so far in physics (exept classical mechanics of course) is always related to light. 
 A: Light exists in nature therefore physicists were bound to be interested in it at some point. But light is special in a sense. All we see, we see with light. Our eyes are sensitive to electromagnetic fields which make up this light. Furthermore, the main force which keeps the stars together is electromagnetism, which is the interaction of matter and light described by Quantum Electrodynamics (QED). Without electromagnetism, gravity would be the only long range force with nothing to oppose it (except maybe the Pauli Exclusion Principle but that a different matter) and matter would crush into itself in a very dense region forming black holes.
On top of that, Einstein discovered that the speed in which light travels, $c$, is an invariant quantity in the universe. From that you can build the whole of special relativity which all modern theories are dependent on.
Like this wasn't enough, General Relativity which is the theory of gravity, also relies on how space-time curves. Space-time, is the merging of space in time into one entity which encodes the relationship of space and time and how it transforms from frame to frame (this is where time dilation and length contraction equations are derived from). Space-time was found to be the correct description of nature, and its was discovered because light as a massless particle always must travels at $c$.
So to sum up, light is special because it was the first massless particle discovered. As described above, this led to a cascade of new discoveries in the last 110 years. We refer to the "speed of light" like light is special but its not. It would probably be better to talk about the "speed of all massless particles", but for historical reasons we don't do that. Thus, in reality, the actual reason in my opinion for which physicists love light is because for many years before the weak and strong interactions were discovered, electromagnetism was the main game in town together with thermodynamics which also involved light.
A: Light is a rare phenomenon as it has no mass. Most particles have mass and therefore cannot act in the way that light is, which is to move at the speed of light. At this speed, due to relativity light moves at the same speed regardless of your perspective; no matter how fast you are able to move you can never catch up to light, and even as you try harder to do so light will always appear to be moving at the same speed away from you.
A: Instead of the word light it would be better to use the word electromagnetism. Newton and Young were fascinated about the decomposition of white light into it colors and about fringes behind edges. Since Maxwell it was obvious that light was only a small part of the electromagnetic spectrum. Later were discovered the weak and the strong nuclear power. But in our life were only the two forces electromagnetism and gravitation we feel every day. About gravitation is only to say that we are subjected to it and that we speculate about the curvature of the space in dependence of masses.
About electromagnetism we know a lot and we use it every day. We know that EM radiation is the result of accelerated particles. Since Lorentz we know how to bind together the three phenomena moving charge, magnetic field and mechanical force. the resulting devices are the electric generator, the electrodrive and the magnetic coil. Laser and LED are EM phenomena too. For the understanding of chemical compounds it is helpful too to know electromagnetism.
There is a lot of physics where you don't need knowhow of EM. but every time you go deeper into some phenomena you end with it. Try it out.
