Interference is an effect that occurs when two waves of equal frequency are superimposed. This often happens when light rays from a single source travel by different paths to the same point. If, at the point of meeting, the two waves are in phase (vibrating in unison, and the crest of one coinciding with the crest of the other), they will combine to form a new wave of the same frequency. The amplitude of the new wave is the sum of the amplitudes of the original waves. The process of forming this new wave is called constructive interference.
If the two waves meet out of phase (crest of one coinciding with a trough of the other), the result is a wave whose amplitude is the difference of the original amplitudes. This process is called destructive interference. If the original waves have equal amplitudes, they may completely destroy each other, leaving no wave at all. Constructive interference results in a bright spot; destructive interference produces a dark spot.
Partial constructive or destructive interference results whenever the waves have an intermediate phase relationship. Interference of waves does not create or destroy light energy, but merely redistributes it.
Two waves interfere only if their phase relationship does not change. They are than said to be coherent. Light waves from two different sources do not interfere because radiations from different atoms are constantly changing their phase relationships. They are non-coherent (see lasers....). [pg. 48-49] [Light and Color, by Clarence Rainwater, Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim, Golden Press, NY, Western Publishing Company, Inc., 1971.]
R E F E R E N C E S
Interference n [1783] 1a: the act or process of interfering b: something that interferes: Obstruction 2: the mutual effect on meeting of two wave trains [as of light or sound] that constitutes alternating areas of increased and decreased amplitude [as light and dark lines or louder and softer sound] 3a: the legal blocking of an opponent in football to make way for the ballcarrier b: the illegal hindering of an opponent in sports 4: partial or complete inhibition or sometimes facilitation of other genetic crossovers in the vicinity of a chromsomal locus where a preceding crossover has occurred 5a: confusion of a received radio signal due to the presence of noise [as atmospherics] or signals from two or more transmitters on a single frequency b: something that produced such confusion 6: the disturbing effect of new learning on the performance of previously learned behavior with which it is inconsistent
Interfere [ME enterferen, fr. MF (s')entreferir to strike one another, fr. OF, fr. entre- inter- + ferir to strike, fr. L ferirrre -more at Bore] [15c] 1: to interpose in a way that hinders or impedes: come into collision or be in opposition 2: to strike one foot against the opposite foot or ankle in walking or running -used esp. of horses 3: to enter into or take a part in the concerns of others 4: to act reciprocally so as to augment, diminish, or otherwise affect one another -used of waves -syn see Interpose
[Merriam-Webster's Collegiate Dictionary, 10th Edition. Springfield, MA, USA: Merriam-Webster, Inc. 1995.]
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