[varsigma]

Just a question:

Have you heard of those guys Fraunhoffer and Fresnel?

[hamdani yusuf (OP)]

Just a question:

Have you heard of those guys Fraunhoffer and Fresnel?

Yes

[hamdani yusuf (OP)]

https://en.m.wikipedia.org/wiki/Fresnel_diffraction

Comparison between the diffraction pattern obtained with the Rayleigh?Sommerfeld equation, the (paraxial) Fresnel approximation, and the (far-field) Fraunhofer approximation

Calling it a diffraction pattern contributed to the widespread confusion. It should be called interference pattern from diffracted light rays.

[hamdani yusuf (OP)]

When the obstruction is opaque enough, we find no diffraction.

It's related to penetration depth, compared to the wavelength. For common metal plates, such as aluminum, penetration depth in microwave frequency is much smaller than the wavelength.
While in visible light, the penetration depth is larger than the wavelength.
Although in both cases, the penetration depth isn't that much different, in the range of microns. The vast difference in wavelength causes the difference in the results.

[varsigma]

The appearance of this thread, in say, the context of a court of some kind, is that of someone making their case about a particular effect, observed in good old nature.

Except, they are stuck it seems, with the one approach. It's classical, this approach. And yet the petitioner, if you will, is trying to understand an entirely local effect. The effect of diffraction of light at the boundary of a material surface, is localized, by ah, the surface.

Material means made out of atoms. Light is made out of particles of light, but a particle isn't a little solid ball of whatever. it just isn't. The classical approach only gets you near to the "surfaces" you desperately want to characterize.

You need to switch donkeys. Read Don Quixote maybe. Study Picasso. Hell, I don't know, get a degree in Quantum Physics.

[varsigma]

Point, 0 dimension.
Edge, 1 dimension.
Surface, 2 dimensions.
Volume, 3 dimensions.

And physics in the classical world we perceive, stops there. Mathematics, and in particular, the mathematics of quantized interaction 'spaces', doesn't stop. Why stop with 4 dimensions? Riemann once asked why stop going backwards once you get to zero? what about -1 dimensions etc?

He was saying we can be moving forward at the end of four dimensions, and moving backward at the beginning, or something, at the end of the day

[hamdani yusuf (OP)]

You need to switch donkeys. Read Don Quixote maybe. Study Picasso. Hell, I don't know, get a degree in Quantum Physics.

Can't you distinguish between interference and diffraction without invoking quantum physics?

[varsigma]

Can't you distinguish between interference and diffraction without invoking quantum physics?

No. At least most modern day physicists will tell you it can't be done. The classical theory of diffraction obviously is a good enough result, to first order, ('chortle').

About Don Quixote and Picasso. Sometimes learning mathematics is like reading one or trying to understand the other.

Then you realize, you are reading one and trying to understand the other. The explanations are straight out of that wonderfully paradoxical story, and the diagrams and equations are Picasso on bloody steroids I tell ye.

[alancalverd]

Can't you distinguish between interference and diffraction without invoking quantum physics?

I can, and have done several times in this thread.

The phenomenon of diffraction can be modelled with a single wavefront and Huygens' construction

Modelling interference requires at least two wavefronts and the principle of superposition

Sometimes, both happen.

[varsigma]

'sigh'

I thought there was a reasonably big clue about measuring an effect. It means you break a symmetry.

So what is the measured difference between interference and diffraction, what are the measurements in either effect so you can say what their difference is? Would that answer the question about the confusion between?

Another problem with light doing this, is that all waves interfere and diffract around barriers. Ocean waves do it, radio waves do it. All waves do it.

[alancalverd]

A nondirectional radio beacon (NDB) is what it says: effectively, a medium-frequency point source in the middle of an airfield. You use a rotating aerial to search for a signal minimum, then fly at 90 degrees to that minimum to reach your destination. VHF ground-to-air communication is likewise usually nondirectional.

Radio waves are diffracted by mountains and other large structures. Flying to an NDB in zero wind, you expect to hold a constant compass bearing. If you descend below the height of an intervening structure, the apparent bearing changes towards the edge of that structure.

If you have two transmitters running at the same frequency, you can encounter periodic maxima and minima, and garbled audio, distributed spatially, thanks to interference.

The solution to diffraction is to fly higher. The solution to interference is to request one transmitter to shut down. It helps to know which problem you are dealing with.

[varsigma]

The solution to diffraction is to fly higher. The solution to interference is to request one transmitter to shut down. It helps to know which problem you are dealing with.

Right. I would say the essential difference is that a single source of wavefronts generates waves that will diffract when they encounter surfaces. Otherwise interference is measured or observed when there are two or more sources.
And being a part-time physicist, I note that radio waves are photons with a long wavelength, so a classical result would be expected and I can ignore the fact that photons are quanta.

Actually that's what a lot of physics does. Perhaps because a classical approximation does work quite well. Nasa doesn't design rockets that have to cope with quantum gravity even though we're pretty sure gravity is quantized.

[alancalverd]

a single source of wavefronts generates waves that will diffract when they encounter surfaces.

Pedant hat firmly in place, I would say "edges" , not "surfaces". Surfaces reflect and/or absorb, but em radiation doesn't "bend" round a surface!

I've been pondering on the photon model of radio waves. We generate them by a continuous process - essentially, sinusoidal alternating current in a wire - with no essential discontinuity of frequency or duration. The energy received from a radio transmitter depends only on how long you listen to it  - it doesn't arrive in discrete packets. Why assume that it is quantised?

[varsigma]

a single source of wavefronts generates waves that will diffract when they encounter surfaces.

Pedant hat firmly in place, I would say "edges" , not "surfaces". Surfaces reflect and/or absorb, but em radiation doesn't "bend" round a surface!

I've been pondering on the photon model of radio waves. We generate them by a continuous process - essentially, sinusoidal alternating current in a wire - with no essential discontinuity of frequency or duration. The energy received from a radio transmitter depends only on how long you listen to it  - it doesn't arrive in discrete packets. Why assume that it is quantised?

Ok sorry, I might have still had my topologist's hat on, where a surface can have less than two dimensions "effectively".

You can treat a barrier like a 1-dimensional 'surface' if it acts as a waveguide, more or less, e.g. a breakwater, but along the surface, not below it. That comes with the idea that it's  some kind of aerial if we are talking radio waves. That 'could' be a gotcha that isn't. A radio broadcast antenna is a 1-dimensional waveguide.

[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.

[alancalverd]

Or maybe because it isn't all that important to most people. To take my example in reply #370 above, the pilot needs to know the difference but 300 passengers, and anyone else who isn't a pilot, doesn't.

[hamdani yusuf (OP)]

Or maybe because it isn't all that important to most people. To take my example in reply #370 above, the pilot needs to know the difference but 300 passengers, and anyone else who isn't a pilot, doesn't.

That's right. Most people don't have to know many things because they rely on someone else to know those things to do the necessary activities for them. Not many people can perform brain surgery, or grow staple food reliably, or build livable houses and bridges, or make comfortable shoes. But someone who does have to do those activities need to know the correct information related to those activities. Imagine mechanical engineers trained using impetus theory.

[alancalverd]

Or civil engineers brought up on Aristotelian mechanics. Like the guys who built the Roman sewers, Hadrian's Wall, Angkor Wat, the Pyramids, Stonehenge, and all that other stuff that has hardly lasted 6000 years.....Rubbish, all of it!

[hamdani yusuf (OP)]

Or civil engineers brought up on Aristotelian mechanics. Like the guys who built the Roman sewers, Hadrian's Wall, Angkor Wat, the Pyramids, Stonehenge, and all that other stuff that has hardly lasted 6000 years.....Rubbish, all of it!

Pyramids builders lived before Aristotle. They used mathematical model good enough to build stationary buildings. Earth curvature and rotation don't have significant effect on their results.

[alancalverd]

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