# What is the difference between diffraction and interference of light?

I know these two phenomena but I want to know a little deep explanation. What type of fringes are obtained in these phenomena?

• – jinawee Sep 29 '14 at 19:18
• I don't understand why this was closed as a duplicate. The core of what's being asked here is very different to the linked questions. – Emilio Pisanty Oct 9 '17 at 15:38
• After all is said and done, it can all be seen as scattering. – Vendetta Jul 10 '18 at 20:15

1. Two separate wave fronts originating from two coherent sources produce interference. Secondary wavelets originating from different parts of the same wave front constitute diffraction. Thus the two are entirely different in nature.
2. The region of minimum intensity is perfectly dark in interference. In diffraction they are not perfectly dark.
3. Width of the fringes is equal in interference. In diffraction they are never equal.
4. The intensity of all positions of maxima are of the same intensity in interference. In diffraction they do vary.

Diffraction pattern

Interference pattern

• Are the fringes wider at the center and narrower at edges in diffraction? – Abdul Wajid Lakhani Sep 29 '14 at 18:38
• +1, nice but this does not hold in general under all cases of diffraction or interference (see next answer as well) – Nikos M. Sep 29 '14 at 18:54
• This is a terrible answer with extremely limited applicability, and it's a shame that it's displayed so prominently on a thread with this many views. The answer by Immortal Player is vastly superior. – Emilio Pisanty Oct 9 '17 at 15:40
• For interference you don't need two different coherent sources: physicsworld.com/cws/article/news/2012/sep/07/… – Ziezi Nov 20 '17 at 23:22
• Ugh. For all the high schoolers reading this: yes, this is what you need to put down on your exam to get full credit. No, real physicists don't actually care about this distinction, which is just splitting hairs. The issue is that you aren't taught enough math in high school to solve any real problems, and the teachers need to test something, so they make up fake vocab words whose definitions you have to memorize. – knzhou Apr 5 '18 at 13:04

Feynman has come from heaven to answer your question! Listen to him:

No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a quest of usage, and there is no specific, important physical difference between them. The best we can do is, roughly speaking, is to say that when there are only a few sources, say two interference sources, then the result is usually called interference, but if there is a large number of them, it seems that the word diffraction is more often used.$_1$

To be more explicit read this passage from Ajoy Ghatak:

We should point out that there is not much of a difference between the phenomenon of interference and diffraction, indeed, interference corresponds to the situation when we consider the superposition of waves coming out from a number of point sources and diffraction corresponds to the situation when we consider waves coming out from an area sources like a circular or rectangular aperture or even a large number of rectangular apertures (like the diffraction grating). $_2$

Credits: $_1$ Feynman Lectures on Physics $_2$Optics-Ajoy Ghatak.

• Yes. The OP asks for a "deep explanation". At a (not-so) deep level, there is no difference at all. – garyp Sep 29 '14 at 18:43
• +1 for relevant quotes. By the way I think the right name is "Ghatak" and not "Chatak". See here: en.wikipedia.org/wiki/Ajoy_Ghatak – Phonon Sep 29 '14 at 20:53
• I would rather say that you use interference to explain diffraction on a slit (Huygens principle) but that does not mean it is the same phenomenon. Interference is just the superposition of waves, doesn't matter how those waves were put together it is much broader subject than diffraction. In diffraction on a slit, the wavefront is being bent, which is explained as interference of elementary spherical waves. One physical phenomenon is used to explain the other. That does not mean they are the same thing. I wrote longer answer below, if you mind discussing. – KabaT Nov 29 '17 at 16:15
• @KabaT: Thank you for the comment. I am now thinking on this: forum.philosophynow.org/viewtopic.php?f=5&t=23045 ; I will be back into physics later. Sorry, now physics is, at current time, is not of utility to me. – Immortal Player Nov 30 '17 at 0:29
• After learning about scattering in physics, I joke about everything being scattering. Reflection, refraction, diffraction, interference, you name it. Of course, we can always try and separate stuff when the problem constraints and physical parameters allow, but... Give me potentials and I'll calculate that in a very unnecessarily complicated manner. – Vendetta Mar 28 '18 at 14:41

Diffraction occurs when a wave encounters an obstacle or a slit these characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength, whereas Interference is the phenomenon where waves meet each other and combine additively or substractively to form composite waves. In a sense there are similarities in the fact that both phenomena from a given wave produce other waves (with in general different frequency or phase and/or amplitude etc.). The main difference is the mechanism, diffraction involves a wave and some obstacle or object which deflects the wave or bends it and intereference involves a wave which combine with other waves. In physical experiments both these phenomena can happen and be part of the same overall phenomenon.

The (geometrical) type of fringes can be similar in some cases to both phenomena or different, i dont think there is a general pattern here (as one can check in wikipedia images).

Diffraction is spreading of the beam of light as it goes through aperture or is emitted from a finite area source. It is due to the fact that the beam of light has some k-vector spectrum that has some finite width.

You can think of it like a bunch of photons having a spread of momenta. It is related to uncertainty principle, because having an aperture confines light in spatial position domain therefore broadening its spatial frequency domain.

Similar effect to diffraction, which happens in space, is dispersion, which happens in time. It makes a light pulse spread in temporal position domain during propagation, due to it having multiple frequencies (in temporal frequency domain) forming it.

However, the causes of these two effects, diffraction and dispersion are a bit different. Diffraction happens because directions of the k-vector spectrum differ, dispersion happens because phase velocity of each frequency differs. Nevertheless equations describing those two phenomena are very similar and for example are leading to the notion of solitons which happen both in space and in time through balancing of dispersion/diffraction with nonlinearity.

On the other hand, interference is a phenomenon resulting from a superposition of waves. They can have different amplitudes, frequencies or phases, and it will influence how the superposed final wave (a sum of all the interfering waves amplitudes) will look like.

This not only is seen in an experiment with two slits in space, but also you can use it to explain forming of ultrashort pulses through constructive interference of waves in some points in time and destructive in other. Two such pulses in close proximity (close here depends on the spectrometer resolution) will also create interference fringes in the measured spectrum.

Interfering waves don't need to be spherical or originate from the same source. If at any point of spacetime some waves of some kind, coming from wherever, will meet, they will interfere in some way or another depending on their parameters.

Diffraction is the tendency of a wave emitted from a finite source or passing through a finite aperture to spread out as it propagates. Diffraction results from the interference of an infinite number of waves emitted by a continuous distribution of source points.

If light is incident onto an obstacle which contains two very small slits a distance d apart, then the wavelets emanating from each slit will interfere behind the obstacle. Waves passing through each slit are diffracted and spread out.At angles where the single slit diffraction pattern produces nonzero intensity, the waves from the two slits can now constructively or destructively interfere.

• This doesn't really go into as much detail as the OP was asking for. – Kyle Oman Sep 29 '14 at 22:27

Both interference and diffraction result from superposition of the EM waves. Inteference result from the superposition of two different coherent sources whereas in diffraction superposition result from different parts of the same source. So we speak about diffraction resulting from a wide slit or circular aperture and interference resulting from two slits or a number of slits. Though light in these slits may originate from the same source we consider them as different sources.

## protected by AccidentalFourierTransformNov 26 '18 at 17:41

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