The Nature of Light

There are two broad classes of light: visible light--light we can see, and invisible light--light we cannot see. Invisible light includes radio waves, microwaves, ultraviolet light, infrared light, x-rays and gamma rays.

The atom is the source of all forms of light, both visible and invisible. Electrons move about the nucleus of an atom in shells or energy levels. When energy is absorbed by electrons, the electrons move to a higher energy level (away from the nucleus). When electrons move to higher energy levels, the atom becomes unstable. Eventually the electron will lose its extra energy and fall back to its original energy level. When the electron releases its absorbed energy, it releases it in the form of a tiny bundle of energy called a photon.

Light is made up of streams of photons. If the electrons release their photons from a very high energy level, the result is high energy light such as x-ray photons. If the electrons release their photons from a moderately high energy level, the result is moderately high energy levels such as visible light. If the electrons release their photons from a low energy level, the result is low energy light such as radio waves.

Even though light varies in its energy level, light of any energy level travels at the same speed. The speed of light in a vacuum is 300,000 kilometers per second. This is a little over 186,000 miles per second. The speed of light is slower when matter such as air or water is present.

In light, electrons are moving from one energy level to another energy level. The movement of electrons is called electricity. In light, electrons have an attraction or a repulsion for other matter. Streams of photons (light) move as a wave. Any form of energy which is a wave form phenomenon is called radiation. Therefore, light is called electromagnetic radiation.

Light waves differ from sound waves in that light waves do not require matter as a medium. Therefore, light can travel in a vacuum, and sound cannot.

The arrangement of electromagnetic waves in order of the wavelength (frequency) is called the electromagnetic spectrum. Waves with the longest waves have the lowest energy. Waves with the shortest wavelength have the highest energy. Gamma rays are very short wavelength, high frequency. Radio waves have a very long wavelength and a very low frequency.

The visible spectrum is the very small portion of the electromagnetic spectrum that we can see. The visible spectrum consists of frequencies of between 430 to 760 trillion hertz. Red is about 430 trillion hertz. Red photons have the lowest energy of visible light. Violet is about 760 trillion hertz. It is the highest energy of all visible light. Red light has long wavelength. Violet light has short wavelength.

Frequencies above or below 430-760 trillion hertz make up the invisible spectrum. Radio waves have the lowest frequency and the longest wavelength in the electromagnetic spectrum. Since radio waves are too low in frequency to be seen or heard, radio waves require a receiver (a radio) to receive and amplify them into audible sounds. Information is transmitted in radio waves by changes in frequency or by changes in amplitude. If this is done by changing amplitude, it is called A.M. for amplitude modulation. If it is done by changing frequency it is called F.M. or frequency modulation.

You should remember that a hertz is one wave or cycle per second. A kilohertz is 1,000 waves or cycles per second. A megahertz is 1,000,000 waves per second. AM radio stations operate on frequencies of 535 kilohertz to 1605 kilohertz. This is why this number range appears on the AM dial of a radio. FM stations operate on frequencies of from 88 megahertz to 108 megahertz. This is why this is the dial range for FM stations.

The radio waves between the frequencies of one billion hertz and one trillion hertz are called microwaves. Microwaves are the highest frequency of radio waves. Microwaves are used for communication and for cooking. The device in a microwave oven which generates the microwaves is called a magnetron tube. Microwaves must lodge in water to produce heat. Without water, heating does not occur. Fortunately, almost everything has some amount of water in it.

Short wavelength microwaves are used in radar. The word radar is actually an abbreviation for RAdio Detection And Ranging device. Radar emits microwaves and detects them as they bounce off of things. A computer then calculates the distance the object is from the radar by the amount of time required to bounce back.

Infrared represents electromagnetic waves with frequencies slightly lower than visible red light. Infrared cannot be seen by people, but they can be felt as heat. All objects give off infrared rays. However, warm objects give off more infrared than cold objects. Infrared motion detectors on a burglar alarm detect the infrared rays given off by a warm body. Some animals can see infrared. Mosquitoes, for example, see infrared.

Ultraviolet represents electromagnetic waves with frequencies just higher than visible violet light. The energy in ultraviolet light is high enough to kill living cells. This is why ultraviolet light from the sun can cause cancer, and why ultraviolet light is used to kill bacteria. However, some amount of ultraviolet light is good for people. Ultraviolet light from the sun triggers skin cells to make vitamin D, a vitamin useful to bones and teeth. This is why Vitamin D is sometimes called the sunshine vitamin.

X-rays represent electromagnetic radiation with frequencies just above ultraviolet. The energy of x-rays is high enough to penetrate many materials. X-rays can easily penetrate such matter such as skin, fat and muscle. However, bones absorb or stop x-rays. This is why x-rays are used medically to see bones. People who work around x-rays shield themselves from these rays using lead. Lead absorbs all x-rays. Being exposed to high levels of x-rays is very harmful to people. All forms of radiation affect fast growing cells more than slow growing cells. This is why radiation is used to kill cancer cells and why ultra sound is preferred to x-rays to view the unborn.

The highest frequency of electromagnetic radiation is called gamma radiation. Gamma rays have the highest energy photons and the shortest wavelength of all electromagnetic waves. Most gamma rays come from outer space. However, some radioactive elements can emit gamma rays. The energy from gamma rays can penetrate 10 feet of concrete

Nature of Light Study Sheet