The light emitted by most light sources contains photons of many different colors. By using a tool that creates a spectrum, scientists can get much more detailed information about the light source than they would get simply by looking at it or taking a picture of the light source. So, how do we make a spectrum, and what is a spectrum?

Here is a very nice photograph of a prism creating a spectrum. In this image, a light source is emitting white light, which includes photons of all colors of the rainbow. As those photons pass through the prism, the change from air to glass and back to air again alters their path. Different photons have their paths altered by different distances, so after they pass through the prism, the red photons hit the screen in one place, the yellow in another, and so on to the end of the screen, where we see the violet photons. This effect is referred to as dispersion.

In the masks that were handed out at the 4th Fest, the plastic film was a special material called a "diffraction grating". This film has very narrow lines etched into its surface. The physics of diffraction gratings is more complex, but the result is the same as when light passes through a prism. The photons of different colors get dispersed, and land on a detector (for example, your eye, a screen, or a camera) in different locations, creating a spectrum on the detector.

When astronomers refer to a spectrum (the plural is spectra), they are usually referring to an image that was made by dispersing the light from a light source onto a camera. Here is an image of the spectrum of an incandescent light bulb. You could simply refer to that image as the spectrum of a light bulb. Here is an image of the spectrum of the Sun. In that image, the spectrum was too long to present as a single narrow strip, so the astronomers broke that strip up into roughly 50 rows. It is not very easy to make careful measurements of the spectra from an object when working with an image, so astronomers often simplify their spectra by making a two-dimensional graph of the spectrum. On the x-axis, they plot the wavelength of the light (which is equivalent to the energy of the light or the color of the light), and on the y-axis they plot the intensity of the light. Here is an example two-dimensional plot of the spectrum of a peculiar star called a blue straggler. Astronomers use the word spectrum to refer either to an image of the light, or the two dimensional plot of the image of a spectrum.