3. Darken the room and turn on the projector. Using tape, hang a piece of holographic diffraction grating over the upper lens of the projector. Before taping, rotate the grating until the best spectra is displayed. Two brilliant visible spectra appear on opposite sides of a line running from the projector perpendicular to the screen. The size of the display can be changed by moving the projector closer or farther away from the screen. Refer to the Analytical Spectroscope activity for more information on how the diffraction grating works. Discussion: Visible light, passing through a prism at a suitable angle, is dispersed into its component colors. This happens because of refraction. When visible light waves cross an interface between two media of different densities (such as from air into glass) at an angle other than 90 degrees, the light waves are bent (refracted). Different wavelengths |
of visible light are bent different amounts
and this causes them to be dispersed into a
continuum of colors. (See diagram.)
Diffraction gratings also disperse light.
There are two main kinds of gratings. One
transmits light directly. The other is a
mirror-like reflection grating. In either case,
diffraction gratings have thousands of tiny
lines cut into their surfaces. In both kinds of
gratings, the visible colors are created by
constructive and destructive interference.
Additional information on how diffraction
gratings work is found in the Analytical
Spectroscope activity and in many physics
and physical science textbooks.
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