LIGHT
 
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What is Light?
What is light? Thats a question that man-kind has been asking itself for centuries. The first person to create a working, accepted theory was Isaac Newton. He proclaimed that light consisted of many "light" particles that traveled in space.

Warning! This is wrong. Please read on for the more correct model for light.

Newton thought this because all the evidence he saw pointed to a particle theory of Light. Every one else believed him because he had been right about so many things that they just trusted what ever he said. There were other theories however but none took on like Newton's particle theory.
In 1678 Christian Huygens found that wave theory could explain the laws of reflection and refraction.
The problem with his theory was that he couldn't explain light's ability to travel without a medium. sound needs a medium to travel. Also if light was a wave then we'd theoretically be able to see around corners because waves diffract.
Huygens' findings wern't enought to change the minds of most scientists who trusted Newton and his particle theory. But in 1801 a scientist named Thomas Young showed that light exibits interference behavior. This made science wonder if maybe light may not be a particle. How could two particles come together and cancel eachother out?
Then in 1850 Jean Foucault showed yet another flaw in particle theory. He found that light traveled at slower speeds in water and glass than in air. Particle theory would say that the effect would be opposite.
Finally Maxwell compared light to electromagnetic waves an found that if Light were infact an electromagnetic wave it would have a speed of approximately 3 x10-7. That in fact is the speed of light.
Hertz and other scientists later tested and proved that light exhibits reflection, refraction, and all other characteristic properties of waves. This showed that light was more like a wave than a particle. What does this mean?!?!?!?
At this point in time, around 1900 the scientific community thought that light must be a wave...This is what happens when you assume that something must fit into a current model! or any model for that matter. Learning phisics isn't about learning numbers, equations, particles,energy or even light....It's about learning how to think beyond the box.
Albert Einstien is probably the most famous person to think "beyond the box." he helped scientists come to the conclusion that Light is niether a wave nor a particle. We'll talk more about him later.
Photoelectric Effect:
Discovered by Hertz, the Photoelectric effect the ejection of an electron from metal exposed to light. An experiment to show this would be to have a circuit in a vaccuum, if the circuit is completed a light will go off, but the circuit is broken. When light is shown on the break in the circuit the circuit is completed and the light shines.

The neat thing about the photoelectric effect is that it is independent of light intensity...thats right, no matter how bright the light is the same amount of electrons leave the metal with the same amount of kinetic energy.

Wait a sec, that sounds like light is a particle...you see that's the problem with light. Light has a dual nature, it acts like a particle sometimes and a wave other times. Fortunally never both at the same time.
Einstien was very interested in light and won the nobel prize for explaining why the photoelectric effect occured. He also used Planks concept of quantization to find that that energy of a photon of light is proportional to the frequency of the electromagnetic wave.
E=hf
This means that the E(nergy) of a photon is equal to the h(planks constant =6.6.-34 J*s) times the f(requency). Here are some methods to use this equation.
Quantization assumes that the energy of a light wave is present in bundles of energy called photons. To quantize something is basically to break it up into countable parts. This Idea is used in many places especially when dealing with the very small or the infinite, such as with calculus.
So as of the 1900's we knew that Light was a wave/particle, we knew it's energy and we knew it's speed.
The speed of light was also hard to find. Galileo tried to find the speed by havind two lanterns being held by people in two watch towers 5 miles away. The idea was to have one lantern open then as the second person saw the light he would open his lantern and Galileo would then time the round trip of the light. This didn't work because light travels at 186000miles per second and 10 miles is nothing compared...the human error completely destroyed the experiment.
Many other scientists tried to find the speed to no avail...Fizeau was the first to do it within acceptable error. He used a rotating, toothed gear, a mirror and a stream of light.
Color:

The color of light is defined by the wave length of the light, for example blue light has a wave length of about 4.55 x10-7 meters whereas red light is 6.22x10-7. Light takes up just a small section of all electromagnetic waves. As you can see from the diagram below.

Electromagnetic Spectrum
When we see an object as a certain color that is because the object only reflects that wavelength. A leaf, for example only reflects light that has a wavelength between 5.77 and 5.97 x10 -7 meters, or 577-597 nano-meters (nm).

So we only see the green because that's the only light that comes back to us from the leaf.
Transparent films only allow a certain wavelength of light to pass through them so when you look through a blue film and every thing has a blue tint it's because only the blue light from every thing is allowed to go through the film onto your eyes.
Now that we know the basics of what light is (or isn't) Let's get into how we use it.
Reflection:
All waves show reflection. In the bathtub, make a ripple and then look at what happened when it hits the side. That's right the wave come right back from where it came from. The same thing happens with light. When you see your self in a mirror, what is that called? A reflection! Another thing you can do at home that is a little bit more interesting is to take a flashlight into a dim room with a mirror, a bathroom would work, point the flash light into the mirror at some object, then look at the real object.
The reason for the light hitting the real object is that the angle of reflection is equal to the angle of incidence. The same angle that the light hits the mirror with it will come out with that same angle.
There's two kinds of reflection: Specular and Diffuse reflection. Spectular reflection is when light hits a very smooth object. When this happens all the light "bounces" off the surface in the same direction and makes a clean reflection.
With Diffuse reflection the light hits a relatively rough surface. This causes the light to be reflected in all directions, shown by a little ball of light coming from the reflected surface.
Refraction:
Refraction is when part of the light goes into the medium. Like when light goes into water from the air. When light is refracted it bends a certain degree depending on the medium. Each medium has a different index for refraction. For example when you look at a fish under water you see the fish in a position other than where it actually is (just try catching one with a stick or your hands). The light that you see bent when it left the water.
Also try sticking a pencil into a glass of water. The pencil looks broke. That's because, again, the light is bent when it leaves the water.

Each medium has an index of diffraction it's determined by the equation:
Different mediums have different indexes of refraction but different wavelengths also have different indexes. When white light hits a prism each wavelength of light bends a different amount causing a rainbow effect.
In fact, that is exactly how rainbows are formed. The White light hits the water vapor in the air and the light separates out into colored light and a rainbow is formed.
Light is one of the amazing phenomena of the universe. To this day we have not mastered the concept of what it truly is. All we know is that it is Light, and that with out it there'd be no life, no sight, and almost no questions for physicists.