Bottom Science
To understand – “what is interference?” we first have a little knowledge about the “Superposition Principle”.
Principle of Superposition
According to the principle of superposition – when two or more sets of waves pass through & cross each other then the resultant displacement (amplitude) produced by several waves at any instant is the vector sum of the displacement (amplitude) produced by each one of the waves.
Interference
Now let’s move ahead and understand what does “interference” actually means.
With a single source of disturbance, the distribution of energy in the surrounding medium is uniform, but when two adjacent sources give out continuous waves of the same wavelength and amplitude and have the same phase (or constant phase difference), the distribution of energy is no longer uniform.
At some points where the crest of one wave falls upon the trough of the other and vice versa, the resultant amplitude is reduced to zero, and energy is minimum.
While at other points where the crest of one wave falls upon the crest (or trough of one wave falls upon trough) of the other, then the resultant amplitude is increased and the energy becomes maximum.
This modification in the distribution of light energy obtained by the superposition of two or more waves is called “interference”.
During the overlap of crest & trough, it should be noted that,
In the phenomenon of interference, there’s only a transfer of energy from one region to other.
The energy missing at one region reappears at another region of the energy distribution of interfering waves, so we can say there’s only redistribution and no destruction and creation of energy in the phenomenon of interference of the waves.
In short, Interference is the redistribution of light energy obtained by the superposition of 2 or more waves from coherent sources (sources that emit light waves with a constant phase difference and nearly equal amplitude).
Condition For Permanent/Good Interference Pattern Of Light
The two sources should continuously emit waves of the same wavelength or frequency, this is because the resultant displacement will be constant only if the periodic time is the same.
For obtaining interference fringes, the amplitudes of the two interfering wave trains should be equal or very nearly equal. This will reduce general illumination (i.e. without it we can’t get completely dark or bright fringe)
The 2 sets of interfering waves should either have the same phase (or constant phase difference), this can only be possible if both waves originate from a single point source because if they originate from a different then it is difficult to handle the fluctuations.
The sources should be very narrow – because if the sources are broad, it becomes equivalent to a large number of narrow sources lying side by side and will produce their own interference pattern that disturbs the fringe system.
The two sources emitting a set of interfering waves should be very close to each other, i.e. the two interfering wavefronts must intersect at a very small angle, otherwise, the path difference between the interfering waves becomes large and interference fringes will get close to each other and will not be sharp.
