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Introduction
What exactly is light? How does it enable us to see things? Light is something that comparatively, we are lucky to have in such abundance. It is one of the most amazing physical aspects of our universe. Light behaves extremely oddly with respect to a number of areas, and without our understanding of the properties of light, we wouldn't be able to manipulate it for the advancement of technology.
Before we get into too much detail regarding how light how it behaves, let's establish a clear understanding of what light is. The transmitting particle of light is called a photon, or quanta. Photons are massless particles, which enable them of course, to travel at the speed of light. According to Einstein's Theory of General Relativity, nothing with mass can travel at or beyond the speed of light. His famous E=mc^2 shows this. That equation shows that mass and energy are really interchangeable. Since "c," the speed of light, is a constant, if E increases, m has to increase. In other words, energy and mass are related in such a way that as the more energy put into accelerating a particle with mass, the more mass the particle gains. To accelerate the particle faster and faster, you would need more and more energy because it would be gaining mass. As you may well know, pushing a car up a hill is a lot harder than pushing a bicycle. So in order for the speed of the particle to reach the speed of light, there would need to be an infinite amount of energy propelling it; this is impossible. This is what sets the speed of light (approximately 180,000 miles per second in a vacuum) as the cosmic speed limit.
When we are dealing with particles that do not have mass, on the other hand, this speed can be reached (but not exceeded). This is what makes massless particles very unique. In fact, photons always travel at the speed of light. A photon has no "rest energy." Without this knowledge, or more importantly, and understanding of why this is so, we could not accomplish a fraction of what we have today with regards to optics, nanotechnology, and other fields as we will later see. For instance, we would not be able to manufacture lenses for glasses, and police officers would not be able to set up speed traps due to the lack of ability to measure the speed of a car (good thing for us though, right?).
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