Natural magnification(special circumstances)

[pallidin]

Hi everyone,

Say, I was wondering if you could explain something to me.
I have two examples to share:

1) I drive by several enclosed bus stops on my way to work. The enclosure consists of a solid roof, but the sides are a metal mesh screen with circular holes less than the diameter of my pinky.
Anyway, as I'm approaching there is one screen facing me and another behind it, say 6-feet from the first mesh.
I can see images of the circular mesh as if it were magnified 500-1000 times!
From my distance, say 100 feet, I can not see the actual circular holes, but I can see, quite clearly, a hugely magnified virtual image.

2) Outside my house right now(which prompted my Q) is a maintenance pickup truck with a diamond shape metal mesh enclosure surrounding the top sides of the pickup bed.
From my distance I can not see the individual diamond mesh, but I see the hugely magnified virtual image. Walking towards it, the mesh is indeed diamond shaped mesh. Again, as in 1), there is a second similar mesh behind it on the other side of the pickup bed.

I assume this has something to do with either interference or the "pin-hole" effect.

Any thoughts?
Thanks...

Edit: sorry for all the edits. Spelling typos.

 

[marcusl]

You are seeing Moire patterns and they are well-studied. They can be understood in many equivalent ways--interference patterns (as you suggested) are a good approach. It can also be understood in a signal processing context as spatial aliasing due to undersampling (violation of the Shannon sampling theorem). There are many Google sites covering these. Please come back if you have further questions!

 

[pallidin]

Thanks Marcus. Moire patterns!
I knew there was an explanation and you nailed it.
Cheers!

 

[sophiecentaur]

You get some interesting Moire patterns through fishnet tights. You may also get a smack.

 

[pallidin]

You get some interesting Moire patterns through fishnet tights. You may also get a smack.

Right, I think I should be most careful as to NOT examine that.

 

[davenn]

Right, I think I should be most careful as to NOT examine that.

chicken ! --- where's your sense of adventure ? ;) haha


But seriously for a moment... does that (Moire patterns) also explain the appearance of what looks like the classical magnetic field lines (bar magnet under paper with iron filings sprinkled on top) that you see around a bright point of light reflecting off say a car's chrome bumper bar etc ?

For many many years, that phenomena has always baffled/intrigued me for an explanation, or am I the only one with weird super vision ;) haha

Dave

 

[pallidin]

chicken ! --- where's your sense of adventure ? ;) haha


But seriously for a moment... does that (Moire patterns) also explain the appearance of what looks like the classical magnetic field lines (bar magnet under paper with iron filings sprinkled on top) that you see around a bright point of light reflecting off say a car's chrome bumper bar etc ?

For many many years, that phenomena has always baffled/intrigued me for an explanation, or am I the only one with weird super vision ;) haha

Dave

Dave, are you seeing this through your windshield or seeing it directly through open air?

Edit: The reason I'm asking is that, a few years ago I bought a newer car with a "perfect" windshield.
In all lighting conditions my viewing was great.

That following winter I used, many times, a scraper to get ice off the windshield(apparently the previous owner had a garage)
Anyway, I noticed a BIG difference when point-light sources came through the windshield after winter.

I assume I incurred multiple scratches on the windshield using my scraper; causing some kind of light interferance.

With my now "newer" car, I no longer use a scraper at all. I just take the time to heat the car up and defrost the windows.

 

[davenn]

good question, its been a while since I saw the effect, prob just not in the right place at the right time.
But from vaguest (sp?) memory being inside a car was irrelevant ... I "thought" it was something I noticed whenever I happened to spy the sun shining off a bright reflective (most likely curved) surface.

I will have to look out for it again and come back with some informed comments :)

Dave