Interference of light is difficult to observe since the waves are so small and the vibrations so rapid. However, interference takes place with any kind of wave motion, and the effects of interference can be more easily seen by the examining the behavior of water waves, which are in many ways similar to light waves, but travel more slowly and are more easily noticeable.
Both these conditions have to be satisfied if two light beams are to add to produce darkness. The two sources of light must both give beams of equal intensity, and both must produce light of the same wavelength. That is, they must be monochromatic.
But although the light from two monochromatic light bulbs will undoubtedly interfere, constructively in some places and destructively in others, this effect will never be visible. The reason for this is that any light source never makes a continuous wave of light. It emits waves in short broken pulses of waves, and no two sources of light will break one pulse and start a new one at the same instant. So even of the ups and downs of the two light beams coincide over one of their short pulses the next pulses will be out of phase, for they will start at different instants and the ups and downs will not coincide. There are so many pulses emitted each second that the alterations in light intensity are too quick to be seen.
Both beams of light must originate from the same source, so that the two sets of pulses start and stop at the same time. They are called coherent. If one of the beams travels a slightly greater distance than the other to the point where they meet, its ups lag slightly behind the ups of the other beam (i.e. get more and more out of phase). When they lag sufficiently for the first wave to have already reached its down when the second is just starting its up, they will obviously interfere destructively.
If the wedge is illuminated with monochromatic or one-color light (a good source of yellow light is common salt burning in a Bunsen flame) the fringes appear as sharp colored and black stripes. However if ordinary white light is used, the fringes are white fringed with the colors of the spectrum, and very few of them can be seen. This is because white light is a mixture of light of different colors (i.e. wavelengths). Since the places on the wedge where the light interferes constructively or destructively depend on the wavelength, the light of different wavelengths in the white light will form dark or colored areas at different places.
Interference fringes can be made a measurable distance apart. Since this distance depends directly on the wavelength of light used, measurement (with a low-power microscope) of the distance between light and dark lines serves as a means of measuring the wavelength of light.
Related categories WAVES AND WAVE PHENOMENA
OPTICS AND OPTICAL PHENOMENA
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