Using a prism, white light from the sun or a light bulb can be split into all the visible colors plus electromagnetic radiation that is invisible to the eye. These absorbing wavelengths are characteristic of the molecule and are called its spectrum . At infrared wavelengths, the spectrum results from vibrations of the atoms in the molecule, while at visible and ultraviolet wavelengths the spectrum is caused by the electrons orbiting the molecule. The spectrum can be calculated from quantum mechanics or measured in the laboratory. Usually, the spectrum consists of many wavelengths, at each of which some percentage of the light can be absorbed.
Some wavelengths absorb more strongly than others.
A laser puts out a single pure color or wavelength, so all its power is concentrated at this single wavelength. When a laser beam goes through a prism, all its light comes out at the same place. The exact wavelength can be tuned slightly by changing the laser temperature or current. The basic laser measurement experiment is fairly simple. The laser light passes through a gas sample and the laser power transmitted through the sample is detected as a function of laser wavelength.