Hobbies And Interests
Young's Experiment
Thomas Young used a single source of monochromatic (single-colored) light and shined it through two slits, projecting the light that emerged from them onto a viewing screen. He observed alternating bands of light and dark on the screen. From this evidence, Young deduced that light must propagate in waves. Many scientists since Young have recreated the characteristic two-point source interference pattern.
Interference
Scientists often describe waves in terms of crests and troughs. Think of water rippling across a pond. At any instant, there will be a crest where the water is high, descending to a trough where it is low, followed by many more crests and troughs. When two waves interact, they add together. If the crest of one wave adds to the crest of another, the result is a larger crest. If the crest of one adds to the trough of another, however, they cancel out. Young interpreted the alternating bands of light and dark as evidence of wave interference. The dark areas occurred where the crest of one wave added to the trough of the other.
Wavelength
In order for the double slit experiment to work, the light must be monochromatic and the waves it produces must be all of the same wavelength. Teachers often repeat Young's experiment as a demonstration in physics classrooms. One variable that can change is the wavelength of light. Red light has a higher wavelength than green light, for example. Students can see that conducting the experiment with red light will produce bands that are a greater distance apart than those green light produces.
Water
A simpler experiment to demonstrate wave interference can give students an excuse to get out of the classroom and head to a nice pond or lake. Throw a single stone into the lake and notice the circular waves that propagate out from the point of impact. Now throw two stones simultaneously into the water, separating them by a short distance. Each will produce the same circular pattern of waves but as the waves overlap, they will interfere with one another. Where the crests of the two waves meet, the resulting crest will be higher. Where a crest meets a trough, the two will cancel out. Throwing multiple stones will produce more waves and their interaction will become increasingly complex.