Spectrum is a band of visible light or any other kind of electromagnetic radiation arranged in order of wavelength. Electromagnetic radiation can be thought of as consisting of waves of electricity and magnetism. Wavelength is the distance between successive wave crests.
A rainbow is a spectrum: Its colors appear in the order of their wavelength. Red has the longest visible wavelength, violet the shortest. The energy of electromagnetic radiation is directly related to its wavelength: the longer the wavelength, the lower the energy. Thus, particles of red light have less energy than particles of violet light. Particles of light are known as photons.
Any object can absorb and emit (give off) electromagnetic radiation. The spectrum of this radiation depends on the substance’s composition and temperature. Thus, by studying the spectrum of an object, scientists can determine its composition and temperature. 
About 1915, the Danish physicist Niels Bohr explained what happens when an atom absorbs or emits electromagnetic radiation. He based his explanation on the most common kind of hydrogen atom, which consists of one proton at the center and one electron in an orbit around the proton. The electron can occupy any of an infinite number of possible orbits. Each orbit is associated with a given level of energy, with lower-energy orbits nearer the proton. When the electron absorbs electromagnetic radiation, it jumps from a lower orbit—one nearer the proton—to a higher orbit. When the electron jumps from a higher orbit to a lower orbit, it emits electromagnetic radiation.
See also Electromagnetic waves; Light (The spectrum of light sources); Rainbow; Spectrometer; Spectroscopy.
