Remember from lesson 11 that vibrating electrons produce an electromagnetic wave. What exactly is an electromagnetic wave? Consider a bar filled with vibrating electrons. a changing electric current is produced around the bar. The changing current in turn produces a changing magnetic field. This field creates a changing electric field, which creates a changing magnetic field, which creates another changing electric field, etc. etc. The electric and magnetic fields constantly regenerate each other to produce an electromagnetic wave. The electromagnetic wave emanates outwards from the source at a constant speed. This speed is the same for all electromagnetic waves, and nothing, nothing can change it. Here is why; if electromagnetic waves were to slow down, the magnetic field would generate a weaker electric field, which would generate a weaker magnetic field, which would generate a weaker electric field etc. etc. until the electromagnetic wave disappeared. This would violate the law of conservation of energy. If the wave sped up, the wave would gain an infinite amount of energy. This would again violate the law of conservation of energy. There is only one speed where thee electric and magnetic fields remain in perfect equilibrium, neither gaining nor losing energy. That speed is 300,000,000 m/s, the speed of light. Light is electromagnetic radiation.

The speed of light doesn't change, but its frequency does. The frequencies of light are arbitrarily broken up into seven classes, which make up the electromagnetic spectrum. Radio wavers have the lowest frequency, followed by microwaves, infrared, visible light, ultraviolet, x rays, and finally gamma rays with the highest frequencies. Visible light only has frequencies ranging from 4.3 * 10¹⁴ Hz to 7 * 10¹⁴ Hz. The lowest frequency of light we can see appears red, and the highest appears violet. As with other types of waves, the frequency of the vibrating source is the same as the frequency of the electromagnetic wave produced.

The way a material reacts when light hits it depends on the frequency of the electrons and atoms in the material. Consider a clear sheet of glass. It is transparent to visible light, meaning that visible light passes through it. It is not transparent to ultraviolet or infrared light. This is because ultraviolet light matches the frequency of the electrons in glass. The electrons vibrate, and the light energy is lost as it becomes kinetic energy. Infrared light matches the vibrational frequency of whole glass molecules. Light energy is again converted into kinetic energy. Visible light does not match the vibrational frequency of either glass electrons or glass molecules. It does not turn into kinetic energy as it passes through glass. Instead, each atom absorbs and then re-emits the light, passing it through the glass. This absorption/emission process takes time, and so light travels through glass more slowly than through air. It is not the speed of light that changes, just the time delay between absorption and emission of the light by the glass molecules and atoms.

Most things are opaque rather than transparent. This means that light is absorbed and not re-emitted by the substance. All the light's energy turns into kinetic energy, and so opaque substances are warmed by light. If an opaque object is placed in the light, some of the light rays will be stopped while others pass on. This creates a dark area behind the opaque object where light can not reach, called a shadow. Large, distant light sources cause sharp shadows, as do small nearby light sources. In most shadows, there is a totally black area, where no light reaches, and a fuzzy black area on the edges, where a little bit of light gets through. The total dark area is called the umbra, and the fuzzy region the penumbra.

The colors of things depend on the colors of light that are reflected by them. A red rose appears red because it reflects red light back to us, while absorbing the other frequencies. If we illuminate the rose with, say, green light, the rose will look black because there is no red light to be reflected. The green light is absorbed. Substances reflect light based on the natural frequency of their atoms and molecules. When the frequency of light matches the natural frequency of the molecules, atoms, or electrons in the substance, the light is absorbed. If not the light is re-emitted. In transparent objects the light is transmitted through the material. In opaque objects, the light is reflected back. An object that reflects all visible frequencies appears white; an object that absorbs all visible frequencies appears black.

When all visible colors of light are added together, white light is produced. The same effect occurs when just red, blue, and green light is mixed. If we add various amounts of red, blue, and green light together, we can produce any color in the spectrum. Red, blue, and green are the additive primary colors. Each additive primary color has a complimentary color. This color is that which, when added to the primary color, produces white light. A little thought will show that the complimentary color of any additive primary is the combination of the other two additive primaries. So for red, the complimentary color is the mixture of green and blue. Green and blue make cyan, green and red make yellow, red and blue make magenta. Therefore, the complimentary color of red is cyan. Red + Cyan = White. The complimentary color of green is magenta. Green + Magenta = White. The complimentary color of blue is yellow. Blue + Yellow = White.

In art class, you learned that red, blue, and yellow were the primary colors. Red and blue paint certainly didn't combine to make magenta paint. The rules for mixing colored pigments are entirely different than the rules for mixing colored light. Cyan, magenta, and yellow are the subtractive primary colors (art teachers usually simplify this to read red, blue, and yellow). When all three are combined, black is produced. Each subtractive primary color has a complimentary color, too. This complimentary color is always the additive primary color that has the subtractive primary color as a complimentary color. So yellow and blue, magenta and green, and cyan and red are complimentary.

Small particles "scatter" light. When a beam of white light from the sun hits a small particle, it's frequencies are scattered in all directions. Higher frequencies are scattered more than lower frequencies, so blue and violet are scattered the most while red is scattered the least. Our eyes are not very sensitive to violet, so we see the sky as blue. Since red is scattered the least, red light travels in the straightest line and goes the furthest distance. In our atmosphere. At sunset, when sunlight has to go the furthest distance through our atmosphere to reach us, only red light can make it all the way without being totally scattered. So we see sunsets as red.