How does diffraction grating create a spectrum?

Answer 1

Let's make certain we are on the same page first. A diffraction grating is an apparatus (piece of plastic, glass, ceramic) that has anywhere from one to ten thousand (or more!) thin grooves cut into it. The grooves act as slits that allow light to pass through the grating at regular intervals. In order to produce a decent diffraction pattern (often called interference fringes), the light must be coherent; that is, the light waves coming from the source must be in phase, containing waves vibrating in the same manner (i.e. rising to a crest and falling to a trough at the same points).

When monochromatic coherent light (think laser light) strikes a diffraction grating, the light spreads out past the slits and shines onto a screen. When the light passes through the slits, some light rays travel straight through, but others travel at an angle away from the slits. When the angled light rays meet other light rays, they interfere and produce either bright spots (antinodes) or dark spots (nodes). If the interfering light rays are in phase, the rays interfere constructively, and the spots are bright; but, due to destructive interference, the spots are dark if the interfering rays are out of phase. With monochromatic light, the diffraction pattern is a series of bright spots separated by dark bands. For instance, if we shone a green laser through a diffraction grating in a dark room, we would see green spots alternating with black bands.

The process is the same for composite light, but the results are a little different. White light is called a composite light because it is composed of six basic colors: redo, orange, yellow, green, blue, and violet. Colors of light are our perception of light waves' frequencies. Since these different colors of light have different frequencies, they also have different wavelengths, and waves with different wavelengths diffract differently. You might remember that longer wavelengths (red light) refract less than short wavelengths (violet light). This pattern is reversed for diffraction: of all the colors of the visible spectrum, red diffracts the most, and violet diffracts the least. Therefore, when white light is shone through a diffraction grating in a dark room, the light waves still spread out past the slits just as monochromatic light does; however, the red light angles away from a slit more than any other color of light, and the violet light angles away the least. The other colors of the visible spectrum fill in the intervening space, and pow!--you have a spectrum! In fact, you should have at least two visible spectra, one on either side of a bright white spot, separated from the white spot by black bands.