Interference and diffraction are closely related concepts, however, they are essentially two different types of waves that originate from different sources. When two waves from different origin points meet, the two separate wavelengths will combine to form one wave. This is known as an interference wave.
When a wave reaches an opening or obstacle, it affects the direction in which the wave is traveling, and the resulting wave is known as a diffraction wave. It is important to note that interference waves only really result when there are one or two wave sources, when there are three or more, the result is almost always diffraction waves.
Interference vs Diffraction
The main difference Between Interference and Diffraction is the appearance of their waves. Interference occurs when the light waves combine through the two different starting points. Whereas diffraction appears due to the superposition of the subordinate wavelengths. The intensity of the edge of interference is always alike. And on the converse, diffraction has odd fringes.
Comparison Table Between Interference and Diffraction (in Tabular Form)
| Parameter of Comparison | Interference | Diffraction |
|---|---|---|
| Number of source points | Two distinct points | Three or more |
| The intensity of wave apex | All apices equal | Varied |
| Width of fringe | Equal | Unequal |
| The intensity of wave trough | Absolutely nothing | Unknown and varied |
| Primary or secondary wave | Always from primary source | Modified from primary wave |
What is Interference?
Interference occurs when two waves emanating from two distinct points interact with one another and combine to create a different waveform altogether.
Two waves that have their peaks and troughs perfectly lined up are said to be “in phase”, and the amplitude of the waves are simply added together to create the resulting waveform.
When the crests of two waves are added together this is known as constructive interference, and the amplitude of the resulting waveform will be the sum of the amplitudes of the crests of the original waves.
When waves are out of sync and the crests and troughs are overlapping, they are said to be “out of phase”.
If the waves are completely out of sync, i.e. one hundred and eighty degrees away from each other, and the amplitudes of the peak and trough of opposite waves are equal, they will cancel each other out in what’s known as destructive interference.
If you think of this in the context of trying to move a large piece of furniture.
If two people were to push from the same end, this is constructive interference as it would create more force than a single person, however, if two people were to push from opposite ends, the furniture would remain still, just as there is no wave amplitude with destructive interference.
In the context of light waves, it is also important to note that an interference wave will show consistent and equal width between light and dark areas when projected onto a screen.
What is Diffraction?
In physics, diffraction is when waves bend around small obstacles, such as sound waves traveling around a corner, or when waves spread out after passing through a small opening.
The secondary waveforms that result after passing through/by the obstacle will be different from the original, with potentially many different and varied phases and amplitudes.
Diffraction only occurs on a significant level when the size of the gap is comparable to that of the wavelength, and given that most wavelengths are very small, the smaller the gap, the more obvious the diffraction.
To picture this, imagine waves of swell from the ocean coming to shore into a narrow rocky opening. Then compare that to waves of swell entering the mouth or a marina.
In the example of waves passing through a narrow opening, you will see the rounded waveform fanning out into the body of water on the other side of the opening, a different shape from the flat waveform that initially entered the opening.
This can be compared to a marina, where we may have larger bodies of water moving from the ocean into the marina, but the size of the opening means that the water inside the marina is barely disturbed by the resulting diffraction waves.
Diffraction waves do not occur when particles pass through slits or around an object, instead, they continue on their original trajectory, unchanged by the external circumstances.
Diffraction waves also have a variety of different peak intensities, due to the many and varied waveforms interacting, as well as the multiple source points (greater than three), that need to be present to result in a diffraction wave.
One interesting phenomenon is that if a diffraction wave passes through two distinct gaps, we will see an interference pattern on the other side, as the two gaps act as two new source points.
Main Differences Between Interference and Diffraction
- Interference waves will arise from two distinct source points combining, whereas diffraction waves derive from three or more.
- The intensities of interference wave apices are uniform and equal, however, those of a diffraction wave is varied and unequal, due to their being the sum of many different waves.
- In an interference wave, the fringe width will also be equal, whereas with diffraction waves we can see inconsistent fringe widths.
- The trough of an interference wave will always be equal to zero, whereas the trough of a diffraction wave can be any number of possibilities due to the multiple combining waves.
- Interference waves also derive from a primary source or cause of the wave, such as a rock thrown into a pool of water, whereas diffraction waves result as secondary waves after the primary passes through an opening or by an object.
Conclusion
The main difference between the two types of waves is in the way that they are both formed. Interference waves are derived from the original wave source, whereas diffraction waves are secondary waves that appear when the wave interacts with an obstacle.
In observing interference and diffraction waves interact with one another we have a tool to be able to further understand the rules of our universe, including in quantum physics with the double-slit experiment.
References
- https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.74.3600
- https://cds.cern.ch/record/396122/files/0521642221_TOC.pdf
- https://ui.adsabs.harvard.edu/abs/1999OptEn..38.1051D/abstract