The interference and diffraction of light, discovered early in the 19th century, soon led to practical tools for measuring the wavelengths of light. The most important tool, still widely used today, is the spectrometer, such as the one shown in FIGURE 37.1. The heart of a spectrometer is a diffraction grating that diffracts different wavelengths of light at different angles. Making the grating slightly curved, like a spherical mirror, focuses the interference fringes onto a photographic plate or (more likely today) an electronic array detector. Each wavelength in the light is focused to a different position on the detector, producing a distinctive pattern called the spectrum of the light. Spectroscopists discovered very early that there are two distinct types of spectra.

FIGURE 37.1 A grating spectrometer is used to study the emission of light.