[hamdani yusuf (OP)]

I googled "what is the difference between diffraction and interference?" and get following result on top:

``Interference'' is the more general concept: it refers to the phenemenon of waves interacting. Waves will add constructively or destructively according to their phase difference. ``Diffraction'' usually refers to the spreading wave pattern from a finite-width aperture.

If I only show pictures category, they give different answers.









Here are some answers from quora:

Two separate wave fronts originating from two coherent sources produce interference. Secondary wavelets originating from different parts of the same wave front constitute diffraction.
The region of minimum intensity is perfectly dark in interference. In diffraction they are not perfectly dark.
Width of the fringes is equal in interference. In diffraction they are never equal.
The intensity of all positions of maxima are of the same intensity in interference. In diffraction they do vary.
When we have two infinitely narrow slits separated by a distance apart near the source, we get interference. But when we have a single slit of finite width or rather an aperture near the source, we get diffraction.

The name interference itself gives it away. Two waves interfere or meet each other. When they do they cause an energy distribution which makes the famous bright and dark bands.

Diffraction refers to the fact that waves can bend around things. Diffraction of sound is so common, that you must have never thought about it. But since light is a wave, it diffracts too.

If you look at the two definitions, it would make one wonder, what’s the meaning of this question. The two phenomenon are so different and so nicely defined, why is there a confusion? Forget differences, what is even the similarity between the two?

Well the confusion is caused by a lot of teachers and books. It’s because of captions given to the images like these




Some are called as interference patterns, and some others are called as diffraction patterns. Which is absolutely non sense. So here is what is really going on.

In each picture, the light is made to pass through slit/s. When they do, the small opening makes the light diffract (bend around and spread out). So there is diffraction happening in all of these pictures. These waves, eventually meet each other and redistribute their energies, ergo, interfere with each other, which we can see as alternate dark and bright regions.
So all patterns (bright and dark regions) are caused by interference of light (by the very definition of it), and in all experiments when slits are used there is diffraction happening at each slit.

So it’s wrong to call something as interference or diffraction pattern. All are interference pattern itself.

Then we have something called the diffraction grating. It’s called so, because it has lots of slits, and so light undergoes lots of diffraction, but again, eventually the pattern is caused by interference of light.

Here is a CD. If someone asks what causes the colouration? If you say diffraction that’s wrong. The pits in the CD act like slits. So in that sense, its called a diffraction grating. But the coloration are caused by interference itself. Where you see Blue, it’s because the blue light is undergoing strong construction and rest, not so much. It’s not so different than the thin film interference, you see in oil spills and soap bubbles


So to summarise, all the energy distribution business, which causes coloration (in white light) or alternate dark and bright bands (in mono chromatic light) are called interference. Whenever waves spread out, or bend around a corner, it’s called diffraction. The two are worlds apart.

And some from stackexchange:

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.
The region of minimum intensity is perfectly dark in interference. In diffraction they are not perfectly dark.
Width of the fringes is equal in interference. In diffraction they are never equal.
The intensity of all positions of maxima are of the same intensity in interference. In diffraction they do vary.

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 2Optics-Ajoy Ghatak.

If this question shows up in a physics exam, what answer should be written by students?

[Colin2B]

The first definition you give is correct:
“Interference'' is the more general concept: it refers to the phenemenon of waves interacting. Waves will add constructively or destructively according to their phase difference. ``Diffraction'' usually refers to the spreading wave pattern from a finite-width aperture.” although I would add that diffraction also occurs at a sharp edge.
The confusion occurs because some of the definitions are referring to diffraction patterns, which are interference patterns from a diffracted wave.
You might like to read this http://www.math.ubc.ca/~cass/courses/m309-03a/m309-projects/krzak/

[hamdani yusuf (OP)]

The first definition you give is correct:
“Interference'' is the more general concept: it refers to the phenemenon of waves interacting. Waves will add constructively or destructively according to their phase difference. ``Diffraction'' usually refers to the spreading wave pattern from a finite-width aperture.” although I would add that diffraction also occurs at a sharp edge.
The confusion occurs because some of the definitions are referring to diffraction patterns, which are interference patterns from a diffracted wave.
You might like to read this http://www.math.ubc.ca/~cass/courses/m309-03a/m309-projects/krzak/

I don't think that diffraction is a subset of interference. We can produce diffraction without interference pattern.



We can also produce interference pattern without diffraction.



I think they are both results of superposition principle. Don't get it wrong. Superposition doesn't necesarily produce interference pattern. If we combine vertically polarized light and horizontally polarized light with the same frequency and amplitude, generally we'll get elliptically polarized light. The eccentricity depends on their phase difference, which can give circularly polarized light as well as linearly polarized light in diagonal orientation.

Lens and mirrors (flat, concave, convex) all work based on superposition principle. Yet they are rarely called interference phenomena.

[alancalverd]

Correct. Diffraction = bending, interference = superposition.

The confusion arises when you model diffraction by the superposition of wavelets, which always struck me as an unnecessary complication.
 

[hamdani yusuf (OP)]

Correct. Diffraction = bending, interference = superposition.

The confusion arises when you model diffraction by the superposition of wavelets, which always struck me as an unnecessary complication.
 

I added into my post after yours. I just want to show that interference is a subset of superposition.  We get interference pattern if the superposition produces darker and lighter area.
I think a contributing factor that adds into the confusion is overreaching analogy of electromagnetic wave using mechanical waves such as sound  and wave on water surface.

[alancalverd]

Your microwave transmitter diagram is important in radio navigation. Some systems use extremely accurate VHF (VOR) and microwave (DME) transmission, where the diffracted signal outside the line of sight is very weak but the range is limited at low altitudes. Older systems (ADF) use medium-wave transmission where the range is excellent right down to sea level but directional accuracy is compromised by all sorts of coastal, terrain and atmospheric "bending".

Not that it matters. Having studied all this and taken the tedious theory and practical exams as a living tribute to the pioneers of navigation, you get your licence, switch on the GPS, and fly or sail direct to your destination - until the time comes to renew your licence!

[Colin2B]

I don't think that diffraction is a subset of interference. We can produce diffraction without interference pattern.

That’s right. As Alan says diffraction is bending, and as your first quote puts it ‘spreading’.

[hamdani yusuf (OP)]

I think that a significant portion of the confusion comes from the assumption that single slit diffraction is the simplest case of diffraction, thus missing the explanation for single edge diffraction.

Let's examine each of the answers available online to identify which answers are the correct ones.


The first point of the answer above is incorrect due to counter example of thin film interference.

The second point is refuted by following counter example.

[hamdani yusuf (OP)]

For the next answer, it seems that they have the label swapped. The blue/left side should be for interference while the red/right side for diffraction.

After the label swap, the top left point is correct.
The top right point has issue with definition of crack. It excludes knife edge/single edge diffraction.

a line on the surface of something along which it has split without breaking apart.
"a hairline crack down the middle of the glass"

The middle and bottom points don't really show the difference between interference and diffraction. But the bottom right point is demonstrably false.
Single slit experiment can produce interference pattern even when the slit width is a thousand times the wavelength, e.g. 0.5 mm slit and 500 nm laser pointer.
In usual single slit experiment, following phenomena occur: diffraction, reflection, interference. Interference pattern is produced by superposition of reflected and diffracted light beams by both edges of the slit.

[hamdani yusuf (OP)]

These answers seems to be describing difference between multiple slits (left side) and single slit (right side) experiment instead.

The same thing happens for the answers below




It seems that those confusions are affected by this answer:

Even this answer is not consistent with the result of multiple slit experiment

[alancalverd]

Alas, Feynman is wrong! Taking the MW radio navigation case as an example, there is only one source and no interference - the signal strength is always the inverse square of distance to the transmitter with no interference maxima and minima - but substantial angular deflection is possible from terrain and atmosphere.

But the confusion is no more important than speed/velocity. The context reveals all, and a pedantic editor will point out any significant ambiguity.

[hamdani yusuf (OP)]

The definition of diffraction above is grossly imprecise. For interference, it seems only specific to mechanical waves while also still imprecise.
The principle is almost correct, although the edge doesn't have to be sharp.
The obstacle part is correct. Interference can happen with our without obstacle, e.g. from two independent sources having same frequency.

[Colin2B]

The principle is almost correct, although the edge doesn't have to be sharp.

The definition is very imprecise. Sharp is relative to wavelength, what is sharp for a light wave is not going to diffract a radio wave or sound wave

[hamdani yusuf (OP)]

The name interference itself gives it away. Two waves interfere or meet each other. When they do they cause an energy distribution which makes the famous bright and dark bands.

The number of waves is not limited to two in order to produce interference pattern.

[hamdani yusuf (OP)]

Definition of diffraction
: a modification which light undergoes especially in passing by the edges of opaque bodies or through narrow openings and in which the rays appear to be deflected

https://www.merriam-webster.com/dictionary/diffraction

The obstacle doesn't have to be opaque to produce diffraction. Transparent materials will do, as I've shown in my experiment.

[hamdani yusuf (OP)]

Another source of confusion I found is inconsistency among sources.
https://en.wikipedia.org/wiki/Diffraction

In classical physics, the diffraction phenomenon is described by the Huygens–Fresnel principle that treats each point in a propagating wave-front as a collection of individual spherical wavelets.[3] The characteristic banding pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength, as shown in the inserted image. This is due to the addition, or interference, of different points on the wave-front (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. However, if there are multiple, closely spaced openings, a complex pattern of varying intensity can result.

The solution provided by the integral theorem for a monochromatic source is:


where U is the complex amplitude of the disturbance at the surface, k is the wavenumber, and s is the distance from P to the surface.

The assumptions made are:

U and ∂U/∂n are discontinuous at the boundaries of the aperture,
the distance to the point source and the dimension of opening S are much greater than λ.

https://en.wikipedia.org/wiki/Kirchhoff%27s_diffraction_formula#Derivation_of_Kirchhoff's_diffraction_formula

[hamdani yusuf (OP)]

I answered the same question in quora. Feel free to comment here to improve accuracy and clarity.
https://www.quora.com/What-is-the-difference-between-interference-and-diffraction/answer/Hamdani-Yusuf

I’ve seen many confusions on these concept which I think need to rectify. We need to clarify what are those concepts, and what are not. Some commonly observed phenomena are combinations of those two. I’ll focus the answer on electromagnetic waves, including light.

DIFFRACTION is the bending of light as it passes around the edge of an object with higher opacity than the medium, towards the area/volume behind that object. The amount of bending angle and intensity of bent light depends on the opacity and shape of the object. Opacity and penetration depth depends on the frequency of light as well as the material of the object.

Non-diffractive obstacle shows that diffraction of light is produced by the obstacle, not the medium. It’s corroborated by vertically tilted diffraction slits that produce conic shaped light beam, which makes the pattern on the screen elliptic or hyperbolic.


In single-slit diffraction, the two sides of the slit act as two sources. If the slit is smaller than the wavelength of light, it effectively becomes a single point source, and no observable interference pattern occurs.


Not diffraction: Bending of light as it passes from one transparent substance into another. It’s called refraction.



Not diffraction: Bending of light at an interface between two different media so that it returns into the medium from which it originated. It’s called reflection.


INTERFERENCE is a phenomenon in which more than one waves combine to form a resultant wave of greater or lower amplitude which are resolved spatially. It’s because different places receive the original waves in different phases. To produce observable interference pattern, the original light waves need to have the same frequencies and similar polarizations.

Interference is a subset of a more general concept, i.e. superposition, which is simply the combination of more than one wave. Below are some examples of superposition which are not generally called interference.

When the amplitude difference is resolved temporally, it is called BEAT. It is caused by slight difference in frequencies of original waves.


When the frequency is significantly different, what we’ll get is color mixing/addition. The variable amplitude is unnoticeable.


When the polarization axes are different, we don’t get visible destructive interference, although they do superpose.

Single slit diffraction is a combination of diffraction, reflection, and interference between diffracted and reflected light wave by the edge of the slit material.

Thin film interference is a combination of refraction, reflection, and interference. No diffraction is involved. Light dispersion occurs in the same experiment when incoming light waves have more than one frequencies.


Another example of interference without diffraction is Lloyd’s mirror. The interference pattern is produced by superposition of reflected light with original lightbeam.


Beam splitter can also produce interference without involving diffraction.


Transmission diffraction grating may involve diffraction when it is made of opaque material. If it’s just a transparent plate with wavy surface, then the phenomena involved are refraction and interference.


Reflection diffraction gratings don’t involve diffraction. Contrary to the name, they involve reflection and interference instead.

[hamdani yusuf (OP)]

Here is another answer I found arguing that Interference and diffraction are really the same phenomenon.
https://www.quora.com/Why-does-interference-happen-during-diffraction

Tipper Rumpf, PhD Optics, University of Central Florida (2006)
Answered April 7, 2018
Interference and diffraction are really the same phenomenon so the question is a little strange. They have slightly different connotations. Diffraction usually describes the spreading or splitting of a wave. Interference describes when there are two or move waves passing through the same location and the waves add or subtract (i.e. constructive or destructive interference).

Perhaps an answer to your question is that diffraction produces waves at different angles and when these overlap you get interference.

Here is another one,

Bill Otto, studied Physics & Chemistry at The University of Alabama in Huntsville (1976)
Answered April 13, 2018
Diffraction and interference are the same phenomena, and there is no clear distinction.

Interference is usually used to characterize the effects of a very small number of slits or beams, while diffraction is used to characterize a large number of slits (such as a grating) or a large area such as the diffraction from from a telescope mirror.

That said, there is no clear distinction between the two, and a discussion of why one is present with the other is moot, unless perhaps in your textbook the terms have been defined differently from the usual definitions.

From The Feyman Lectures[1]

the name has been changed from Interference to Diffraction. No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them. The best we can do, roughly speaking, is to say that when there are only a few sources, say two, interfering, 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. So, we shall not worry about whether it is interference or diffraction, but continue directly from where we left off in the middle of the subject in the last chapter.

https://www.feynmanlectures.caltech.edu/I_30.html

[hamdani yusuf (OP)]

Jess H. Brewer, Physics professor since 1977.
Answered May 11, 2018
They are pretty much the same thing. The way we calculate single-slit diffraction is by breaking the slit up into an infinite number of infinitely small slits right next to each other and calculating the resultant interference pattern.

468 viewsView Upvoters · Answer requested by Bashir Ahme

Vaibhav Sharma, Physics PhD Student at Cornell University
Answered February 3, 2014
Originally Answered: what is the diffrence between interference and diffraction ?
In reality, there is no difference. According to Huygen's principle, each point is a source of a secondary wavelet. In both the phenomena, secondary wavelets interact and build up a pattern of bright and dark fringes. In interference, we consider the case where there are 2 or multiple slits and where the size or the aperture of the slit is ignored and only the distance between the two slits is taken into account. In diffraction, there is only one slit and thus it's size or aperture is taken into account.

For interference, we consider mainly the interaction among wavelets coming from the two different slits by ignoring the size of the slit and assuming that only one wavelet is emitted from each slit. For diffraction, we consider the interaction among the different wavelets coming from the different portions of the same slit by accepting the fact that the slit is not just a point source but it has a finite aperture with many points. In reality, both diffraction and interference take place simultaneously.

It seems that the confusion has been widely spread even among academics. It may happened because experiments most frequently introduced to demonstrate those phenomena involve both of them simultaneously, such as single slit and double slit experiments. Showing experiments which demonstrate each phenomenon independently will help a lot in recognizing the difference.

[hamdani yusuf (OP)]

A answered another question in Quora, Why are there dark fringes in a single slit experiment?
https://www.quora.com/Why-are-there-dark-fringes-in-a-single-slit-experiment

Because in a single slit apperture, there are two edges. They can act as light sources which then interfere constructively and destructively to create dark and bright fringes on a screen. Double slit apperture has 4 edges, hence there are more light sources which can interfere with each other.


Top picture is the pattern produced by the same double slit aperture as the bottom picture, but one slit is covered.

According to Babinet principle, single slit apperture and a thin wire produce similar interference patterns.

If you think that single slit interference pattern is produced by the space between edges of the slit which act as wave sources, you’ll be in trouble explaining the same pattern produced by a thin wire.