Introduction to the Electromagnetic Spectrum
What does the word light make you think of: the sun, a lamp, the many colors of the rainbow? Light is but a small part of a very broad family of mostly invisible waves. Electromagnetic waves form a continuous spectrum of wave energy ranging from very long radiowaves to very short gamma ray waves. Visible light represents only a very small portion of this spectrum.
You are surrounded by waves, both visible and invisible. Below is a diagram that shows a portion of this electromagnetic spectrum. Notice the small section that constitutes the small portion that is visible to the human eye. Note, however, that we can detect a portion of the infrared with our skin. The ultraviolet rays can easily give us a bad sunburn in the summer. Even if we don’t feel light, we can feel its effects (as can be said for all shorter wavelengths — they damage living cells).
Electromagnetic waves are transverse waves composed of alternating electric and magnetic fields. They are created by accelerating charges or changing magnetic fields. Electromagnetic waves can travel through a vacuum and, unlike sound waves, they do not need a medium to travel through. All magnetic waves travel at the speed of light. The wavelength and the frequency of electromagnetic waves vary depending on the portion of the electromagnetic spectrum being investigated.
Wavelength is the distance between two equivalent parts of the wave (two troughs or two crests). The unit of measure is the meter and the symbol is lambda.
Frequency is the number of waves that pass a point in one second. When the wavelength is short, the frequency is high because more waves pass through a point in one second. The unit of measure for frequency is the hertz.
The electromagnetic spectrum shows increasing frequency and decreasing wavelength as you go from left to right. Radiowaves include AM , FM radio and shortwave radio. On the spectrum, television waves and radar waves come after radiowaves, but before microwaves. Infrared waves are just to the left of visible spectrum and can’t be seen with the human eye. Next come the visible spectrum which contains all the colors of light seen by the human eye. To the right of the visible spectrum are the ultraviolet waves. Then come the X-rays, which are used to photograph dense material such as metal welds, bones and some kinds of animal tissues. Gamma rays have the shortest wavelength and the highest frequencies. Also note that the energy transmitted and the probability of damage to living tissue increase as you move to the right on the spectrum.
Radiowaves: We are all familiar with AM and FM radio and shortwave. FM waves carry the picture portion of most television shows. The sound of most television shows is carried by AM waves.
Microwaves: High-energy radiowaves are called microwaves. They are used in communications. They reflect off certain surfaces and are absorbed by others. These absorbed microwaves can be used for cooking.
Infrared waves: Infrared waves are slightly longer than visible red light. They are used by certain types of cameras to show heat.
Visible spectrum: Each color in the visible spectrum — red, orange, yellow, green, blue, indigo and violet — has a different frequency. Red, green and blue are primary colors that can be combined to create other colors.
Ultraviolet rays: The main source of ultraviolet rays is sunlight. They can be used to destroy bacteria and viruses. They also can cause sunburn and skin cancer.
X-rays: Most people have had experience getting pictures of their bones or teeth taken through the use of X-rays.
Gamma rays: Radioactive material and nuclear reactions give off gamma rays. They damage living cells and are sometimes used to destroy cancer cells.