The Electromagnetic Spectrum
Light is a form of electromagnetic radiation. Electromagnetic waves travel at the speed of light and can have almost any frequency or wavelength. The distribution of electromagnetic radiation according to its frequency or wavelength (or energy) is the electromagnetic spectrum. The electromagnetic spectrum is the continuous distribution of frequencies of electromagnetic radiation ranging from approximately 105 Hz (radio waves) up to greater than 1020 Hz (x-rays and gamma rays). Equivalently, it is the distribution of wavelengths of electromagnetic radiation ranging from very long ( λ = 106 meters, radio waves) to the very short wavelengths of x-rays and gamma rays ( λ = 10-15 meters). Note that the higher frequencies correspond to lower wavelengths and vice versa ( ν = c/ λ). Finally, the electromagnetic spectrum can also be separated according to the photon energy of the radiation, ranging from 10-29 joules (radio waves) up to 10-14 joules (x-rays and gamma rays). Note that photon energy increases with increasing frequency (E=h ν).
The electromagnetic spectrum can be divided into regions which exhibit similar properties, each of which itself constitutes a spectrum: the x-ray spectrum, the ultraviolet spectrum, the visible spectrum (which we commonly refer to as "light"), the infrared spectrum and the radio-frequency spectrum. However, these divisions are arbitrary and do not imply a sharp change in the character of the radiation. The visible light spectrum, while comprising only a small portion of the entire electromagneticspectrum, can be further divided into the colors of the rainbow as was demonstrated by Newton. The other regions of the electromagneticspectrum, although invisible to our eyes, are familiar to us through other means: x rays expose x-ray sensitive film, ultraviolet light causes sunburn, microwaves heat food, and radio frequency waves carry radio and television signals.
The interaction of electromagnetic radiation with matter is studied in the field of spectroscopy. In this field, spectra are used as a means to graphically illustrate which frequencies, wavelengths, or photon energies of electromagnetic radiation interact the strongest with the material under investigation. These spectra are usually named according to the spectroscopic method used in their generation: nuclear magnetic resonance (NMR) spectroscopy generates NMR spectra, microwave spectroscopy generates microwave spectra, and so forth. In addition, these spectra may also be named according to the origin or final fate of the radiation (emission spectrum, absorption spectrum), the nature of the material under study (atomic spectrum, molecular spectrum) and the width of the electromagnetic spectrum which undergoes the interaction (discrete, line, continuous, or band spectrum).