When light passes through a glass prism the angle at which it is deflected depends upon its wavelength. This feature of prisms makes it possible to separate light into its component wavelength. Water droplets do the same thing to create rainbows.

The optical device which incorporates a prism for separating and measuring the wavelength of the component parts of some light source is called a spectroscope. When physicists began using the spectroscope to analyze the light from objects so hot that they glowed some interesting regularities were observed. The plot of the intensity of light versus the wavelength (or frequency) showed a peak with the intensity dwindling to minute amounts for wavelengths of light above or below the wavelength at which the peak occurred.