[hamdani yusuf (OP)]

But they are all subject to gravitational and meteorological influences, which are still not fully understood!

That's a reason why we need to improve our understanding of physical reality, by pointing out limitations of our models.

[alancalverd]

Why bother? They have all outlived their supposed usefulness - apart from the Roman sewers!

[hamdani yusuf (OP)]

That's a reason why we need to improve our understanding of physical reality, by pointing out limitations of our models.

Why bother? They have all outlived their supposed usefulness - apart from the Roman sewers!

Because unexpected results come from false assumptions. We don't know what problems are waiting for us to solve in the future. And wrong decisions might bring unacceptable consequences.

[alancalverd]

There's an old adage that "thermodynamics owes more to the steam engine than the steam engine owes to thermodynamics".

A great deal of science begins with an engineering failure, and we question our hypotheses in the light of the unexpected finding, because that shows us where to look!

[hamdani yusuf (OP)]

When we found that someone else is using the wrong model of the universe to make decisions, we should at least tell them that there is a better model, and describe the situation where the difference between those models become significant.

-- Donald Trump supporters are interviewed in Rome, Georgia, revealing that their reasons for supporting Trump are a confusing mishmash of misunderstandings and confusion
---

[alancalverd]

When we found that someone else is using the wrong model of the universe to make decisions, we should at least tell them that there is a better model, and describe the situation where the difference between those models become significant.

.....and join the ranks of Bruno, Galileo, Solzhenitsyn, Navalny......Even if your discovery would benefit those in power, you have to face down all the criticism of your peers as Einstein and Darwin did. 

[hamdani yusuf (OP)]

When we found that someone else is using the wrong model of the universe to make decisions, we should at least tell them that there is a better model, and describe the situation where the difference between those models become significant.

.....and join the ranks of Bruno, Galileo, Solzhenitsyn, Navalny......Even if your discovery would benefit those in power, you have to face down all the criticism of your peers as Einstein and Darwin did. 

That's why you need strategies. Direct confrontation doesn't always give the best results.

[hamdani yusuf (OP)]

This lecture explicitly show double slit experiment as an example of interference, and single slit experiment for diffraction. If left uncorrected, the confusion will spread among students. To make it worse, they will get positive feedbacks in the exams by giving wrong answers.

[alancalverd]

This lecture explicitly show double slit experiment as an example of interference, and single slit experiment for diffraction.

What would you expect from a guy who can't spell diffraction? 

[hamdani yusuf (OP)]

This lecture explicitly show double slit experiment as an example of interference, and single slit experiment for diffraction.

What would you expect from a guy who can't spell diffraction? 

I don't think that language is a major obstacle in understanding scientific explanations. Especially when online translators are freely available.

[Eternal Student]

Hi.

If left uncorrected, the confusion will spread among students. To make it worse, they will get positive feedbacks in the exams by giving wrong answers.

1.   It's not in my power to make all students and teaching staff use the terminlogy only a certain way, sorry.

2.    Some of the important ideas for diffraction and interference are the same.   Blending the terms may not be as bad as you think.   Scientific terminology evolves just like all spoken languages evolve and develop.   If it became essential to point out some differences between one thing and another, there are other ways and other terminology we can use.

Evolving terminology isn't always bad:
       In Britain, the word   "inflammable" meant that something could easily be set on fire or was some kind of fire hazard.   The etymology of the word is from the Latin inflammabilis.    However, English started to evolve in a way where adding "in" as a prefix in front of a word indicated the negation or opposite.   
Examples:   Hospitable  and  Inhospitable;      Tolerable   and   Intolerable;   Edible and Inedible......  plus many others.

As a consequence some people thought that "inflammable" meant that something would NOT burn and indeed, when surveys were done in some workplaces, a few people thought they might be able to use an inflammable material to put a fire out if one started somewhere.   Obviously this is a serious problem.   So the resolution was to change the terminology assocated with something that will burn or ignite easily easily.   HazChem warnings now say  "Flammable"  instead of "Inflammable".     This was probably a senisble decision and may have saved lives.    Technically, it's wrong on many levels, the Latin roots have been lost and the word "Flammable" was just nonsense that wasn't in any dictionary when the decision was made.   However, it was still probably the right decision.

Best Wishes.

[alancalverd]

Some of the important ideas for diffraction and interference are the same. 

I think students should be taught the difference between "one" and "many", or even "two", however unfashionable such pedantry may be.

[hamdani yusuf (OP)]

Evolving terminology isn't always bad:

It isn't always good nor acceptable either.
What's the benefit of confusion between diffraction and interference?

[hamdani yusuf (OP)]

Some of the important ideas for diffraction and interference are the same.

I think students should be taught the difference between "one" and "many", or even "two", however unfashionable such pedantry may be.

Some people think that single slit experiment is the simplest example of diffraction. They seem to forget that a slit has two edges. Diffraction can be produced by one edge, which is simpler than two.

Interference is better observed by the formation of dark regions caused by destructive interference. It requires at least two sources with similar frequency and polarization.

[alancalverd]

Diffraction is a phenomenon associated with a single source of radiation.

Interference is a different phenomenon which requires (by definition) at least two sources.

What is the problem, for heaven's sake?

[hamdani yusuf (OP)]

Fresnel Diffraction at Straight Edge

Shadow of straight edge at a large distance. Fresnel diffraction pattern.

Experiment like this may contribute to the confusion. Destructive interference seems to occur even when there is only one edge. Don't forget that diffracted light rays can interfere with unobstructed light rays. Just like in Lloyd's mirror, where reflected light rays interfere with direct light rays.

[hamdani yusuf (OP)]

My answer to the question in the title is because they have learned from erroneous primary sources. And they were not equipped with necessary tools to distinguish between the correct and incorrect explanations.

By being freely accessible, Wikipedia articles have been the primary sources for many people. When diffraction is defined as interference there, how can they be distinguished from each other?

Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.
https://en.m.wikipedia.org/wiki/Diffraction

[alancalverd]

Time for you to write to Wikipedia. "Aperture" adds nonsense to the confusion: an aperture doesn't have a shadow!

[hamdani yusuf (OP)]

Time for you to write to Wikipedia. "Aperture" adds nonsense to the confusion: an aperture doesn't have a shadow!

It says geometrical shadow. Which simply means regions behind the aperture uncovered by geometrical rays from the light source.

[alancalverd]

Then why doesn't it say so?