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Section 5: The Electromagnetic Spectrum
Figure 3-8. A Prism Makes Visible the Rainbow of Colors Present in White Light
A triangular glass prism, similar to the one used by Sir Isaac Newton, can be used to divide white light into its constituent colors.
© Science Media Group.
In 1666, Sir Isaac Newton (1642–1727) used a prism to split sunlight into its many colors, and introduced the term "spectrum." This was the first step toward using light as a tool for exploring the inner workings of the atom.
Matter is capable of absorbing or emitting light, and chemists can detect and use this light to understand what is happening on the atomic level. The study of matter by means of the light that it gives off or absorbs is called "spectroscopy." The field of spectroscopy began in 1814 with the invention of the spectroscope, a device for seeing and measuring the brightness of types of light. The spectroscope was invented by German optician Joseph von Fraunhofer (1787–1826). (His work is described in more detail in the sidebar.)
Figure 3-9. Joseph von Fraunhofer in His Laboratory
A spectroscope has an optical system that isolates a source of electromagnetic radiation and then splits that source into its component frequencies, allowing the relative brightness of each to be compared and recorded. Most spectroscopes are calibrated to known light sources; modern-day spectroscopes are essential tools in chemical analysis, in astronomy, and in industry.
© Public Domain, Wikimedia Commons.
However, the Sun produces other wavelengths of light, which are invisible to the human eye, and unsurprisingly these forms of light were discovered much later than visible light. In order to categorize light, we need to understand that light is the more common name for "electromagnetic radiation." Also, all electromagnetic radiation can be described as being a wave, and thus has interesting properties that come from that. There are three main properties of electromagnetic radiation that can vary: energy (Ε), frequency (ν), and wavelength (λ). Wavelength and frequency are inversely proportional; long wavelengths correspond to high frequencies and vice versa. Energy is directly proportional to frequency; the higher the frequency, the larger the energy. Gamma rays are high-energy waves that are emitted during certain types of nuclear reactions, whereas radio waves have long wavelengths and are completely safe to humans.
In addition to gamma rays and radio waves, two other forms of light exist: infrared (which has a longer wavelength than visible light) and ultraviolet (which has a shorter wavelength than visible light). In fact, the electromagnetic spectrum is a continuum that includes radio waves at the long wavelength end, through microwaves, infrared, visible light, ultraviolet, X-rays, and, finally, to gamma rays at the shortest wavelength end. Nonetheless, one characteristic of all types of radiation is that they have the same speed in a vacuum, approximately 3 x 108 meters per second, which is equivalent to about 700 million miles per hour.
Figure 3-10. Electromagnetic Spectrum
A diagram of the electromagnetic spectrum showing the regions of the spectrum in order by increasing energy (decreasing wavelength). Notice that as the wavelengths get shorter, the frequency of the waves (how often the waves pass) increases .
While we often refer to radio signals by their frequencies, it's more customary to refer to visible light by its wavelength. Knowing the speed of light, and the wavelength of the light under consideration, allows the calculation of its frequency. Visible light occupies the part of the electromagnetic spectrum from 400 to 800 nanometers (nm), where a nanometer is one-billionth of a meter. The longest visible wavelengths—800 nm—are red, and the shortest—400 nm—are violet; wavelengths shorter than 400 nm are ultraviolet; those longer than 800 nm are infrared.
The shorter the wavelength, the more energetic is the emitted energy. For example, a mixture of hydrogen and oxygen gas will not explode when exposed to visible light, but shining ultraviolet light on a container of oxygen and hydrogen will trigger the reaction. Likewise, exposing one's skin to bright light by itself will not cause skin damage, but exposure to ultraviolet light can cause tanning, sunburn, and in some cases skin cancer.