[hamdani yusuf (OP)]

Aluminum is a conductor, paraffin wax is a dielectric, so you wouldn't expect them to behave the same.

Aluminum produces diffraction in visible light spectra, just like paraffin wax.

I'm preparing a new video investigating diffraction of light by producing one sided interference pattern. One edge of the single slit aperture is made of total internal reflection barrier, while the other edge is made of folded aluminium foil as a normal barrier.

[hamdani yusuf (OP)]

It's been 3 years since I started this thread. But still, the confusion doesn't seem to go away.
Here's what I got from Google search just now.

Interference is a property originated by waves from two different coherent sources, whereas secondary wavelets that originate from the same wave but occur from different parts of it, produce the phenomenon termed diffraction.

And the next top searches are:

https://dewwool.com/difference-between-diffraction-and-interference/
Diffraction and interference are phenomenons associated with the wave nature of particles. Diffraction can be plainly defined as the spreading of waves while passing through obstacles. Interference can be defined as the combination of two or more waves. In this article, we discuss 10 differences between diffraction and interference.

https://collegedunia.com/exams/difference-between-diffraction-and-interference-physics-articleid-1566
Interference is a feature caused by waves from two independent coherent sources, whereas Diffraction is caused by secondary wavelets that originate from the same wave but occur in various areas of it. It's essential to understand the fundamental differences between them by looking at the region of least intensity; in interference, this region is extremely dark, but in diffraction, it's less dark. There are a few other criteria that distinguish diffraction from interference, in addition to these few distinctions which are tabulated below.

https://www.toppr.com/guides/physics/difference-between/diffraction-and-interference/
Important difference between diffraction and interference

Parameter   |   Diffraction   |   Interference
Occurrence   |   It occurs because of the secondary wavelengths superposition   |   It occurs due to the light waves superposition that is from two sources
Width of fringes   |   Unequal   |   Equal
Intensity of fringe   |   Not same, in case of diffraction, for all fringes   |   Same fringe intensity for all the fringes
Obstacle or slit   |   There is a requirement for it   |   There is not a requirement for it
Fringe spacing   |   Non-uniform in the case of diffraction   |   Uniform in the case of interference
Contrast between maxima and minima   |   The contrast between maxima and minima is poor   |   The contrast between maxima and minima is certainly good
Wave propagation direction   |   It changes after diffraction   |   It does not change after superposition

https://www.vedantu.com/physics/difference-between-diffraction-and-interference
Interference

Interference is the phenomenon that takes place when the meeting of the two waves takes place as they travel along with the same medium. Besides this, the interference causes the medium to take a particular shape or orientation. Moreover, this shape is due to the whole or the net effect of two individual waves on the medium’s particles.

Diffraction
Diffraction happens due to the spreading out of waves passing through an aperture. In the case of diffraction, the size of the obstacle or aperture is of similar straight dimensions to the incident wave’s wavelength, and its occurrence is significant. Furthermore, it takes place when the traveling wavelength’s part gets obscured or shaded.

[alancalverd]

You have chosen some very bad sources. The English is poor and the physics is awful.

[hamdani yusuf (OP)]

You have chosen some very bad sources. The English is poor and the physics is awful.

Nevertheless, they are chosen by Google's algorithm.
Some say that AI is simply statistic in steroid. Whatever repeated most often, will come up as most significant factor in final results. I hope this issue catches the eyes of popular science communicators, so the vicious circle of spreading misconceptions can eventually be stopped.

[Eternal Student]

Hi.

It's been 3 years since I started this thread. But still, the confusion doesn't seem to go away.  Here's what I got from Google search just now.

    Are you really surprised by this?   You might be seriously over-estimating what a thread in this forum can do or should be trying to do.

    Google (and people in general) don't usually check this small science forum website to get a definition of what the terms "interference" and "diffraction" should be or to obtain an explanation for the subtle differences between them.   
   I'm not criticising your thread or your ideas at all, actually it's quite interesting and shows you have spent a lot of time thinking over some stuff.    However, if there was any scientific term I wanted a definition for,  let's say the "Schwarzschild radius", then I wouldn't start by looking at what some members on a small science forum had to say about it.

    None-the-less, if you are trying to change the use of these terms in the wider community then I wish you the best of luck.   
    At some point in the thread you might have told us why you feel the terms need to be kept so separate but I can't find that on scanning through the thread.  Perhaps you could tell us again.   In particular, is there a change to any physics that follows from something or is it just that you dislike two terms being used inconsistently or interchangeably in some situations?

Best Wishes.

[alancalverd]

Nevertheless, they are chosen by Google's algorithm.

There's the problem.
Google rates the most-viewed sites highest. The English idioms in the passages you quoted are distinctly Indian and appear to have originated in a text aimed at schoolkids - possibly an online classroom. So it will have had an enormous number of readers and no real editing . One obvious giveaway was the use of "medium" - electromagnetic waves can interfere in vacuo!

IMHO school science texts are not edited by scientists but by "communicators" who insist on replacing my three old chums A, B, and C with ethnically balanced gender-neutral names and don't give a damn about physics.

Far better to turn to Wikipedia, which is effectively peer-reviewed and refined by people who know what they are talking about.

However you have clearly demonstrated one important problem: beware of any easy consensus based on poorly-stated or poorly-understood science!

[alancalverd]

At some point in the thread you might have told us why you feel the terms need to be kept so separate but I can't find that on scanning through the thread.  Perhaps you could tell us again.

Diffraction occurs at an edge or a point and results in a downstream wavefront with the same wavelength as the original source, appearing to originate from that edge/point.

Interference is the result of superposition of two or more wavefronts producing maxima and minima with a spatial distribution related to the distribution of the sources. 

If you stand in the shadow of a mountain you may hear the radio signal of a station that is just out of your line of sight. The signal strength you receive decreases smoothly as you move away from the station or further "behind" the mountain. Diffraction. This can cause serious radio navigation errors.

If you have two stations transmitting  the same signal, the intensity you receive will vary sinusoidally as you move in any direction  relative to them. Interference. This was used for very precise long range radio navigation systems such as "Knickebeine" and "LORAN".

[Eternal Student]

Hi.

    Yes, OK, it's possible to try and keep diffraction and interference separate.
However,   is it necessary to try and do so    and    is it accidentally concealing the possibility that they are both due to the same underlying phenomena?

    Let's take an example and consider Huygens principle of secondary wavelets.   It's not the best ever explanation of how or why em radiation travels and exhibits certain phenomena,  for example you can always ask why the secondary wavelets only travel forward,  however it is one reasonable model that can be used.
    Let's just assume that something like Huygens principle is happening.   According to many references (e.g. see any reference provided below), the continued propagation of a plane wave or spherical wave can be explained just by using Huygens principle.   Specifically, once a wavefront has travelled away from it's original source (any tiny distance) then it is possible to forget about that source entirely.   The next position of the wavefront at a later time, t+δt, can be determined just by considering every point on the wavefront at time t to act as a new spherical source.   This is an important point so it's worth saying it again in a slightly different way...
    Nature does not need to know or be concerned about what the original source of the wave might have been.   It is not trying to keep the wave progressing or propagating away from that original source.  As far as Nature is concerned there is now a set of sources (one new source at every point on the wavefront) and the new position of the wavefront will be determined just by propagating each of those spherical waves away from those sources.
   A simple version of Huygens principle is provided by reference [2],  where it is stated that the new wavefront would be on the surface which is tangential to all the spherical wavelets.   Other references (e.g. [3] ) make the situation more easily expressed by referring to "the envelope" of the spherical wavelets but this is the same surface.  I think - but I'm not certain - that the original statement by Christiaan Huygens used the concept of an envelope but it probably wasn't in English and the translation is subject to some interpretation.
   Now, most versions of Huygens principle suggest that the secondary sources do behave like genuine sources of a new wave in most respects.   In particular there will be some superposition and interference.   It is very possible that the placement of the new wavefront on the envelope of the set of spherical wavelets is just a simplification of the underlying process that is actually happening.  Examples:
a)    From reference [1]:   ...every point on a wavefront is itself the source of spherical wavelets, and the secondary wavelets emanating from different points mutually interfere. The sum of these spherical wavelets forms a new wavefront.
b)    From reference [4]:   Wave propagation is linear so superposition holds: it should be possible to decompose an impulsive propagating wave front into its constituent points, then consider the impulsive wavelets radiating from each of those points at a future time, and combine those wavelets in a simple direct geometric manner to obtain the progressing wave front at that future time.

    I have included reference [4] just because it's one of the few articles I've seen where a very direct attempt is made to remove the notion of placing the new wave front "on the envelope" and there is an attempt to show that the new wavefront could just be a result of something like conventional superposition and interference.   There is a different type of wave generated by Huygens secondary sources (a Dirac delta function) but with that assumption you no longer need to exclude a backward propagation, there is just an automatic cancellation when a sum of all the waves is computed.  In a similar way, there is no effect often described as a "wake" left behind.   What remains from the superposition is precisely the same as putting the new wavefront right on the envelope (i.e. a lot of the problems and apparently arbitary rules for Huygens principle have been resolved).
   Anyway the key issue is that what is seen as a conventional straight line or spherical propagation of a wave could actually be the result of interference from many secondary wave sources on the wavefront.

Let's take this point mentioned in one of the much earlier posts:

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

   
   It is quite possible that you are seeing the results of interference.  The propagation of those waves just doesn't look like a typical interference pattern but wherever there is straight line or spherical propagation of a wave then this has been the result of interference from Huygens secondary spherical wavelets.   More-over the "diffraction" effect from the knife edge on the mountain is entirely due to the phenomena of Huygens secondary wavelets.   The radio waves are entering the geometric shadow only because there were no wavelets below them that were able to provide the interference necessary to keep the radio wave progressing in a straight line from the original transmission aerial.

Summary:
   This is NOT an attempt to state that the propagation of an e-m wave has to be this way.  I have provided reference [4] because it's an article in a respected publication ("Nature", see * LATE EDITING) and it simply suggests that Huygens principle and ideas along the lines of conventional superposition and interference could play an important role in all wave propagation phenomena.
   As such what we call "diffraction" could very well be just an "interference" effect.  When light seems to bend around an obstacle, that could just be due to the lack of the interference required to keep it going in a straight line from the original source.  The opening sentence of this post can be re-stated:   Is it necessary to keep the terms "diffraction" and "interference" entirely separate    and    is it accidentally concealing the possibility that they are both due to the same underlying phenomena?


Best Wishes.

References:
    1.    Wikipedia:    https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle
    2.   Byjus:    https://byjus.com/physics/the-huygens-principle-and-the-principle-of-a-wave-front/
    3.   University of New Mexico, physics course:   https://physics.unm.edu/Courses/Fields/Phys2310/Lectures/lecture16.pdf
    4.   Nature*:   https://www.nature.com/articles/s41598-021-99049-7

* LATE EDITING:   On closer inspection, reference [4] is properly an article from "Scientific Reports" which is a sub-journal affiliated to "Nature".   It still has a peer review process but the readership and general academic reference value is not the same as the major journal "Nature".

[hamdani yusuf (OP)]

is it accidentally concealing the possibility that they are both due to the same underlying phenomena?

they are both due to the same underlying phenomena called superposition.

[alancalverd]

It is important to distinguish between them because they have different causes and different effects, as I pointed out in reply #66.

The diagram from HY's contribution is bizarre as it shows the diffracted signal as having a variable frequency, roughly twice that of the incoming signal! That's a pity because it prevents us seeing interference between the primary and diffracted wavefront which he has actually demonstrated in some of his videos. 

Huygens gives us a mathematical model from which we can predict both phenomena.

Beware of byjus. I have no idea where it originates - the idioms appear to be Indian - but it is generally of poor quality.

The UNM lecture opens with a key statement, the precise details of which are correct and all too frequently overlooked

 

We are going to talk about a way to visualize wave
propagation that is controversial in its exact physical
interpretation.

.....yes, it is a mathematical model of propagation - nothing more or less. And a very good one, because it also allows us to predict the outcome of interference.

Part of the problem of taking the model too seriously is summarised in the Nature article - you have to invent some physical process for eliminating the backward wave, and this can lead to all sorts of nonsense about virtual dipoles in space, compression of the aether, etc, whereas Maxwell's equations address the physics and only generate the forward vector that actually happens.

Yes, I'm a pedant, but if we don't distinguish between

de-fuse and diffuse (listen to any newsreader)
hypo and hyper
m and M
force, power and energy

we may end up killing someone. And if you can't distinguish diffraction from interference when approaching a coast at night, you can end up killing yourself. 
 

[alancalverd]

they are both due to the same underlying phenomena called superposition.

NO!!! there is no superposition in diffraction, because there is only one wavefront!!!!

That, in a nutshell, is the whole point of the argument!!!

[hamdani yusuf (OP)]

At some point in the thread you might have told us why you feel the terms need to be kept so separate but I can't find that on scanning through the thread.  Perhaps you could tell us again.

I've summarized the topic into a short video in another thread.

This is my first video entry. The time limitation is real pain in the process. I did what I can to meet it without losing much of important information.
Let me know what you think.

PS. If someone plays the video in this page, will it count as a Youtube view?

[hamdani yusuf (OP)]

Google (and people in general) don't usually check this small science forum website to get a definition of what the terms "interference" and "diffraction" should be or to obtain an explanation for the subtle differences between them.

The differences are conceptual, and they are not by any mean subtle. IMO, the confusion occurred because the most popular experiments to demonstrate those phenomena involve both of them. Phenomena which show them separately don't seem to get much attention, but they are exactly what we need to clear up the confusion.
Here's my unshrink video, I hope you find it easier to follow.

[hamdani yusuf (OP)]

they are both due to the same underlying phenomena called superposition.

NO!!! there is no superposition in diffraction, because there is only one wavefront!!!!

That, in a nutshell, is the whole point of the argument!!!

Diffraction does involve superposition, as it requires macroscopic objects which in turn are made of multiple particles. The direction of propagation of resulting wave depends on the position of the particles, as well as the phase of their oscillations. The training video from Royal Canadian Air Force that I've posted earlier shows that clearly.

[alancalverd]

The direction of propagation of resulting wave depends on the position of the particles, as well as the phase of their oscillations.

Poppycock.

The frequency of a radio homing beacon is usually around 400 kHz. How fast do you think the atoms of the white cliffs of Dover are oscillating? Or are you thinking about the blades of grass on top?  Wavelength of 750 m is rather longer than an atom or even a tall stem of wheat. If the phenomenon had anything to do with  interference, the maxima would be 1.5 km apart - utterly useless for navigation.

What you call a "conceptual difference" is what scientists (and parents) call "giving different names to different things".

[hamdani yusuf (OP)]

 In particular, is there a change to any physics that follows from something or is it just that you dislike two terms being used inconsistently or interchangeably in some situations?

Inconsistency is the meaning of words may cause miscommunication, and we don't know for sure when will it cause catastrophic events. Miscommunication about measurement units has been recorded in history as the cause of some devastating events.
In educational world, when misconception is widespread among the educators, then students who know the correct concept and give the correct answers in an exam could be jeopardized.

[hamdani yusuf (OP)]

The direction of propagation of resulting wave depends on the position of the particles, as well as the phase of their oscillations.

Poppycock.

The frequency of a radio homing beacon is usually around 400 kHz. How fast do you think the atoms of the white cliffs of Dover are oscillating? Or are you thinking about the blades of grass on top?  Wavelength of 750 m is rather longer than an atom or even a tall stem of wheat. If the phenomenon had anything to do with  interference, the maxima would be 1.5 km apart - utterly useless for navigation.

What you call a "conceptual difference" is what scientists (and parents) call "giving different names to different things".

I wrote that interference and diffraction are distinct phenomena, but both are effects of superposition.

[evan_au]

(400 kHz homing beacon)...interference and diffraction are distinct phenomena, but both are effects of superposition

The behaviour of radio waves from a homing beacon can be described quite well by Maxwell's equations. This is "classical" physics.
- Superposition derives from quantum theory, and cannot be described by classical physics.
- You don't need superposition to describe radio waves

[hamdani yusuf (OP)]

(400 kHz homing beacon)...interference and diffraction are distinct phenomena, but both are effects of superposition

The behaviour of radio waves from a homing beacon can be described quite well by Maxwell's equations. This is "classical" physics.
- Superposition derives from quantum theory, and cannot be described by classical physics.
- You don't need superposition to describe radio waves

Superposition principle has been widely used long before quantum theory.
https://en.wikipedia.org/wiki/Superposition_principle

The superposition principle,[1] also known as superposition property, states that, for all linear systems, the net response caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually. So that if input A produces response X and input B produces response Y then input (A + B) produces response (X + Y).

A function F(x) that satisfies the superposition principle is called a linear function. Superposition can be defined by two simpler properties: additivity


and homogeneity

for scalar a.
This principle has many applications in physics and engineering because many physical systems can be modeled as linear systems. For example, a beam can be modeled as a linear system where the input stimulus is the load on the beam and the output response is the deflection of the beam. The importance of linear systems is that they are easier to analyze mathematically; there is a large body of mathematical techniques, frequency domain linear transform methods such as Fourier and Laplace transforms, and linear operator theory, that are applicable. Because physical systems are generally only approximately linear, the superposition principle is only an approximation of the true physical behavior.

The superposition principle applies to any linear system, including algebraic equations, linear differential equations, and systems of equations of those forms. The stimuli and responses could be numbers, functions, vectors, vector fields, time-varying signals, or any other object that satisfies certain axioms. Note that when vectors or vector fields are involved, a superposition is interpreted as a vector sum. If the superposition holds, then it automatically also holds for all linear operations applied on these functions (due to definition), such as gradients, differentials or integrals (if they exist).

[hamdani yusuf (OP)]

Interestingly, the article also mentions the difference between wave diffraction vs. wave interference

https://en.wikipedia.org/wiki/Superposition_principle#Wave_diffraction_vs._wave_interference
Wave diffraction vs. wave interference
With regard to wave superposition, Richard Feynman wrote:[2]

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.

Other authors elaborate:[3]

The difference is one of convenience and convention. If the waves to be superposed originate from a few coherent sources, say, two, the effect is called interference. On the other hand, if the waves to be superposed originate by subdividing a wavefront into infinitesimal coherent wavelets (sources), the effect is called diffraction. That is the difference between the two phenomena is [a matter] of degree only, and basically, they are two limiting cases of superposition effects.

Yet another source concurs:[4]

In as much as the interference fringes observed by Young were the diffraction pattern of the double slit, this chapter [Fraunhofer diffraction] is, therefore, a continuation of Chapter 8 [Interference]. On the other hand, few opticians would regard the Michelson interferometer as an example of diffraction. Some of the important categories of diffraction relate to the interference that accompanies division of the wavefront, so Feynman's observation to some extent reflects the difficulty that we may have in distinguishing division of amplitude and division of wavefront.

The Fyenman's quote above doesn't seem to show his own opinion, but rather his observation on how those words were used by his peers.