Light
Light:
Light is the only thing we can see. Many things generate light in many different ways, through photons. In 1905, Albert Einstein considered the photoelectric effect. This means that when ultraviolet light hits a surface and causes electrons to be emitted from the surface. Einstein�s explanation for this was that light was made up of a stream of energy packets, called photons. Modern physicists believe that light can behave as both a particle and a wave. It is easiest to talk about light as waves, because this provides the best explanation for most of the phenomena our eyes can see. Why is it that a beam of light radiates outward? What is really going on? To understand light waves, it helps to start by discussing a more familiar kind of wave: the one we see in the water. One key point to keep in mind about the water wave is that it is not made up of water: The wave is made up of energy traveling through the water. If a wave moves across a pool from left to right, this does not mean that the water on the left side of the pool is moving to the right side of the pool. The water has actually stayed about where it was. It is the wave that has moved. When you move your hand through a filled bathtub, you make a wave, because you are putting your energy into the water. The energy travels through the water in the form of the wave. All waves are traveling energy, and they are usually moving through some medium, such as water. A water wave consists of water molecules that vibrate up and down at right angles to the direction of motion of the wave. This type of wave is called a transverse wave.
Light waves are a little more complicated, and they do not need a medium to travel through. They can travel through a vacuum. A light wave consists of energy in the form of electric and magnetic fields. The fields vibrate to the direction of movement of the wave and to each other. White light is composed of electromagnetic vibrations. If the intensity of these vibrations is strong, the light is white; if the intensity is less, the light is gray; and if the intensity is zero, the light is nonexistent, or black. The size of a light wave is measured as its wavelength, which is the distance between any two corresponding points on successive waves. The wavelengths of the light we can see range from 400 (violet) to 700 (red) billionths of a meter. The color of light of a single wavelength or of a small band of wavelengths is known as a pure spectral color or hue. Such pure colors are said to be fully saturated and are seldom encountered outside the laboratory. An exception is the light of the sodium-vapor lamps used on some modern highways, which is almost fully saturated spectral yellow. The wide varieties of colors seen every day are colors of lower saturation, that is, mixtures of light of various wavelengths. Hue and saturation are the two qualitative differences of physical colors. The quantitative difference is brilliance, the intensity or energy of the light.
Light not only vibrates at different frequencies, it also travels at different speeds. Light waves move through a vacuum at their maximum speed, 300,000 kilometers per second or 186,000 miles per second. Light waves slow down when they travel inside substances, such as air, water, glass, or a diamond. The way different substances affect the speed at which light travels is key to understanding the bending of light, or refraction. So light waves come in a continuous variety of sizes, frequencies, and energies. We refer to this continuum as the electromagnetic spectrum.
Photons:
Any light that you see is made up of a collection of one or more photons propagating through space as electromagnetic waves. In total darkness our eyes are actually able to sense single photons, but generally what we see in our daily lives comes to us in the form of zillions of photons produced by light sources and reflected off objects. If you look around you right now, there is probably a light source in the room producing photons and objects in the room that reflect those photons. Your eyes absorb some of the photons flowing through the room, and that is how you see. There are many different ways to produce photons, but all of them use the same mechanism inside an atom to do it. This mechanism involves the energization of electrons orbiting each atom's nucleus. An electron has a natural orbit that it occupies, but if you energize an atom you can move electrons up to higher orbitals. A photon of light then gets produced whenever an electron in a higher-than-normal orbit falls back to its normal orbit. During the fall from high-energy to normal-energy, the electron emits a photon -- a packet of energy -- with very specific characteristics. The photon has a frequency, or color, that exactly matches the distance that the electron falls.
There are cases where you can see this phenomenon quite clearly. For example, in lots of factories and parking lots you see sodium vapor lights. You can tell a sodium vapor light because it is very yellow when you look at it. A sodium vapor light energizes sodium atoms to generate photons. A sodium atom has 11 electrons, and because of the way that they are stacked in orbitals one of those electrons is most likely to accept and emit energy. The energy packets that this electron is most likely to emit fall right around a wavelength of 590 nanometers. This wavelength corresponds to yellow light. If you run sodium light through a prism, you do not see a rainbow, you see a pair of yellow lines. Probably the most common way to energize atoms is with heat, and this is the basis of incandescence. If you heat up a horseshoe with a blowtorch, it will eventually get red hot, and if you heat it enough it gets white hot. Red is the lowest-energy visible light, so in a red-hot object the atoms are just getting enough energy to begin emitting light that we can see. Once you apply enough heat to cause white light, you are energizing so many different electrons in so many different ways that all of the colors are being generated -- they all mix together to look white, as explained in one of the sections below.
The thing to note from this list is that anything that produces light does it by energizing atoms in some way.
Color:
Visible light is light that can be perceived by the human eye. When you look at the visible light of the sun, it appears to be colorless, which we call white. And although we can see this light, white is not considered to be part of the visible spectrum. This is because white light is not the light of a single color, or frequency. Instead, it is made up of many color frequencies. When sunlight passes through a glass of water to land on a wall, we see a rainbow on the wall. This would not happen unless white light was a mixture of all of the colors of the visible spectrum. Isaac Newton was the first person to demonstrate this. Newton passed sunlight through a glass prism to separate the colors into a rainbow spectrum. He then passed sunlight through a second glass prism and combined the two rainbows. The combination produced white light. This proved conclusively that white light is a mixture of colors, or a mixture of light of different frequencies. The combination of every color in the visible spectrum produces a light that is colorless, or white.
So, there are two basic ways by which we can see colors. Either an object can directly emit light waves in the frequency of the observed color, or an object can absorb all other frequencies, reflecting back to your eye only the light wave, or combination of light waves, that appears as the observed color. For example, to see a yellow object, either the object is directly emitting light waves in the yellow frequency, or it is absorbing the blue part of the spectrum and reflecting the red and green parts back to your eye, which perceives the combined frequencies as yellow.
Light Hitting Objects:
When a light wave hits an object, what happens to it depends on the energy of the light wave, the natural frequency at which electrons vibrate in the material and the strength with which the atoms in the material hold on to their electrons. When this happens, light can either be reflected off the object, be scattered by the object, be absorbed by the object, be refracted through the object, or pass through the object with no effect. Also, more than one of these things can happen at once.
Transmission:
If the frequency or energy of the incoming light wave is much higher or much lower than the frequency needed to make the electrons in the material vibrate, then the electrons will not capture the energy of the light, and the wave will pass through the material unchanged. As a result, the material will be transparent to that frequency of light. Most materials are transparent to some frequencies, but not to others. (Example: gamma rays and X-rays will pass through ordinary glass, but ultraviolet and infrared will not.
Absorption:
In absorption, the frequency of the incoming light wave is at or near the vibration frequency of the electrons in the material. The electrons take in the energy of the light wave and start to vibrate. What happens next depends upon how tightly the atoms hold on to their electrons. Absorption occurs when the electrons are held tightly, and they pass the vibrations along to the nuclei of the atoms. This makes the atoms speed up, collide with other atoms in the material, and then give up as heat the energy they acquired from the vibrations. The absorption of light makes an object dark or opaque to visible light. (This is why black shirts seem to absorb heat and white shirts reflect heat.)
Reflection and Scattering:
The atoms in some materials hold on to their electrons loosely. In other words, the materials contain many free electrons that can jump readily from one atom to another within the material. When the electrons in this type of material absorb energy from an incoming light wave, they do not pass that energy on to other atoms. The energized electrons merely vibrate and then send the energy back out of the object as a light wave with the same frequency as the incoming wave. The overall effect is that the light wave does not penetrate deeply into the material. In most metals, electrons are held loosely, and are free to move around, so these metals reflect visible light and appear to be shiny. A reflected wave always comes off the surface of a material at an angle equal to the angle at which the incoming wave hit the surface. In physics, this is called the Law of Reflectance. You can see for yourself that reflected light has the same frequency as the incoming wave. Just look at yourself in a mirror. The colors you see in the mirror's image are the same as those you see when you look down at yourself. The colors of your shirt and hair are the same as reflected in the mirror as they are on you.
Scattering:
Scatering is merely reflection off a rough surface. Incoming light waves get reflected at all sorts of angles, because the surface is uneven. The surface of paper is a good example. You can see just how rough it is if you look at it under a microscope. When light hits paper, the waves are reflected in all directions. This is what makes paper so incredibly useful -- you can read the words on a printed page regardless of the angle at which your eyes view the surface. Another interesting rough surface is Earth's atmosphere. You probably don't think of the atmosphere as a surface, but it nonetheless is "rough" to incoming white light. The atmosphere contains molecules of many different sizes, including nitrogen, oxygen, water vapor and various pollutants. This assortment scatters the higher energy light waves, the ones we see as blue light. This is why the sky looks blue.
Refraction:
Refraction occurs when the energy of an incoming light wave matches the natural vibration frequency of the electrons in a material. The light wave penetrates deeply into the material, and causes small vibrations in the electrons. The electrons pass these vibrations on to the atoms in the material, and they send out light waves of the same frequency as the incoming wave. But this all takes time. The part of the wave inside the material slows down, while the part of the wave outside the object maintains its original frequency. This has the effect of bending the portion of the wave inside the object toward what is called the normal line, an imaginary straight line that runs perpendicular to the surface of the object. The deviation from the normal line of the light inside the object will be less than the deviation of the light before it entered the object. The amount of bending, or angle of refraction, of the light wave depends on how much the material slows down the light. One interesting note about refraction is that light of different frequencies, or energies, will bend at slightly different angles. Waves with higher energies take longer to interact with the object, so it is slowed down more than waves with lower energies. This is why waves with higher energies are bent to a greater degree.
Conclusion:
So in conclusion, everything we see is a product of, and is affected by, the nature of light. Light is a form of energy that travels in waves. Our eyes are attuned only to those wave frequencies that we call visible light. Intricacies in the wave nature of light explain the origin of color, how light travels, and what happens to light when it encounters different kinds of materials.