Light
Light is a form of energy, and exists in two conceptual frameworks: light exhibits properties that have characteristics of discrete particles (eg. energy is carried away in "chunks") and characteristics of waves (eg. diffraction). This split is known as duality. It is important to understand that this is not an "either/or" situation. Duality means that the characteristics of both waves and particles are present at the same time. The same beam of light will behave as a particle and/or as a wave depending on the experiment. Furthermore, the particle framework (chunks) can have interactions which can be described in terms of wave characteristics and the wave framework can have interactions that can be described in terms of particle characteristics. The particle form is known as a photon, and the waveform is known as electromagnetic radiation. First the photon…
A photon is the light we see when an atom emits energy. In the model of an atom, electrons orbit a nucleus made of protons and neutrons. There are separate electron levels for the electrons orbiting the nucleus. Picture a basketball with several sizes of hula-hoops around it. The basketball would be the nucleus and the hula-hoops would be the possible electron levels. These surrounding levels can be referred to as orbitals. Each of these orbitals can only accept a discrete amount of energy. If an atom absorbs some energy, an electron in an orbital close to the nucleus (a lower energy level) will jump to an orbital that is farther away from the nucleus (a higher energy level). The atom is now said to be excited. This excitement generally will not last very long, and the electron will fall back into the lower shell. A packet of energy, called a photon or quanta, will be released. This emitted energy is equal to the difference between the high and low energy levels, and may be seen as light depending on its wave frequency, discussed below.
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The wave form of light is actually a form of energy that is created by an oscillating charge. This charge consists of an oscillating electric field and an oscillating magnetic field, hence the name electromagnetic radiation. We should note that the two fields are oscillating perpendicular to each other. Light is only one form of electromagnetic radiation. All forms are classified on the electromagnetic spectrum by the number of complete oscillations per second that the electric and magnetic fields undergo, called frequency. The frequency range for visible light is only a small portion of the spectrum with violet and red being the highest and lowest frequencies respectively. Since violet light has a higher frequency than red, we say that it has more energy. If you go all the way out on the electromagnetic spectrum, you will see that gamma rays are the most energetic. This should come as no surprise since it is commonly known that gamma rays have enough energy to penetrate many materials. These rays are very dangerous because of the damage they can do to you biologically (See the HSW article entitled "How Nuclear Radiation Works" for a further discussion of gamma radiation.). The amount of energy is dependent on the frequency of the radiation. Visible electromagnetic radiation is what we commonly refer to as light, which can also be broken down into separate frequencies with corresponding energy levels for each color.