Starship Asterisk*

Please do not attempt to respond to this thread to discuss the challenge image send to APOD by MIT Physics Professor Walter Lewin.

CHALLENGE AT APOD
OFFICIAL LEWIN'S EXPLANATION

Is it the same diffraction that occurs when you view the sun using a screen with a small aperture to reflect onto another surface?
Are the colored rings caused by the suns corona refracting the same way the moon's corona does through ice particles?

It would appear the photo is taken facing away from the sun, with reflected and refracted sunlight being returned from some transparent (or translucent) particles. If this is the case then we need to identify what particle shape, and material would produce a ring of this apparent size.

My guess is that the foreground is you holding up a flashlight, above you is some kind of tarp with a mesh. It is wet. Behind the mesh is a sculpture which reflects the light unevenly, or maybe the bright area is from the bulb of the flashlight and the lens dims the outer part of the bright area. Water scatters the blue so we see blue first, then the other colors in the rainbow in the outer ring. Is this close?

AdiabaticDannis

newtons' ring

I'm pretty sure that the stuff on the ground is the little retroreflective glass beads that they put down on things like road stripes and signs to make them show up better in headlights at night. They go under brand names such as VisiBead.

The actual effect, I assume, is very similar to the one in this old APOD: http://antwrp.gsfc.nasa.gov/apod/ap040806.html

A circular diffraction pattern. Instead of sunlight, the source of light is the camera flash. The background is likely Visqueen membrane often used at construction sites to protect from moisture at the end of the work day. A nightly rain or heavy dew combined with the either a translucent Visqueen or a dusty transparent Visqueen would provide the diffraction substance.

The bright source of light in the center is the interference pattern maximum. The ring seen is the first minima. The shorter wavelengths of light (Red/Orange/Yellow) are diffracted further. The longer wavelengths (Blue Indigo Violet) are diffracted less.

Of course, its been a few years since I taught hight school physics, so I'm a bit rusty!

Oops, my URL didn't come out. It should've been:

They go under brand names such as VisiBead.

I believe the halo is produced by sunlight directly behind the camera man
reflecting, refracting off of fine glass beads spread on the pavement. These glass beads are frequently used in highway lane marking paint. They make the lines appear to glow when headlights shine on them

ahecht has the correct answer! traffic paint beads spilled on the ground at a highway construction site. The bright inner halo is the white light reflected from the inside of the beads. The rainbow is the refracted light created at the correct angle for the refracting light into the colors. Outside the rainbow halo, the light white light is reflected away from the viewer's eye.

It would appear that Lewin casts a saintly shadow. I'd guess the order & type of colors seen at the halo fringe must be indicative of rank. (did I win ?)

looks like someone is welding something

I'm rather surprised none of the APOD editors could recognize the phenomenon given in the 13 September 2004 photo when one considers that they published a similar event on 6 August!

The photo shows a form of "glory." Typically, a glory is seen from above, opposite the sun and centered on the viewers shadow. Typically (again), it results from sunlight being diffracted, refracted and reflected back to the viewer by water droplets in the atmosphere.

In the case of the 13 September picture, it would appear that some rather strategically-scattered sand or fine quartz on the pavement is serving as the refractive/diffractive/reflective material. (Correspondents writing of reflective beads may be on the mark.)

Presumably, the order of colors present is dependent on the refractive index of the particles in combination with particle-size based diffraction characteristics.

I think I've seen photos like this before. The one I particularly remember is a picture of an airplane's shadow inside the anti-corona. Looked very very much like this, with the light source behind the observer and a ring of colors.

This appears to be an example of a Brocken spectre and Glory (http://www.sundog.clara.co.uk/droplets/glory.htm, http://www.sundog.clara.co.uk/droplets/heilig.htm). The heligenshein is explained at http://www.sundog.clara.co.uk/droplets/heilfrm.htm

This could be as simple as a reflective surface down on the ground and the light shining from behind the photograhper therefore causing the circular light and the it's spectral ring, but what would be the reason for the entire area not being of the same color scheme?

My guess is undecided. my explination above is as good a guess as any, but my hypothesis would have to be that the photographer would need to explain this to show what it is, because one guess is as good a guess as any.

I don't think that this would be the same sort of thing as the August 6th posting, the dark form seems much to defined to seem like something like the moon (in this dark picture) wouldn't be able to show it so cleanly.

I for one can't wait for the answer.

I have a different suggestion:
The surface imaged appears to be cement. I believe the refraction occurs in a layer of warm air sitting on top of the surface (the same thing that causes roads to be reflective on hot days), and the reflection is the natural albedo of the fine grainy cement. This granularity could cause an effect that the direction opposite the sun is a lot brighter than all other regions, the same effect as on the moon. In this layer of warm air, light reflected from the anti-solar point at a certain angle will escape but at a certain angle, different for every wavelength, the light will exprience total reflection and not make it out. This might account for the circular halo.

Somehow I do not believe that the airplane/halo/gegenschein explanation is correct. After all, the APOD editors denied that they were able to figure out the phenomenon, yet linked to that very picture within the text of today's APOD.

I am a Boeing 707 pilot and I see these rings on a regular basis whenever I go fly. It normally occurs on a bright sunny day when the shadow of my jet is cast on a flat blanket of clouds a few thousand feet below me. If the clouds are close enough, the outline of my jet can be quite crisp, but as the clouds become more distant the shadow will eventually disappear and actually become the brightest point in the middle of the colored rings. I always figured it is kind of like a magnifying glass/concave lens. If you put an object directly between a lens and the surface where its light is cast you can still completely eliminate the objects shadow when placed at the right distance. So, you may ask how does an airplane act as a lens? Well, it doesn't, it is like a prism. Since the light is cast all the way around my jet the result is a circle, much like a lens. And since the light is dispersed like it would be by a prism the resulting colors of violet, blue, green, yellow, orange, and then red can be seen. The colors in Prof Walter Lewin's picture aren’t so identifiable because of the poor surface of which he took the picture on. However, on a pure white cloud the colors are much more crisp. If you look very closely at the professor’s picture though, the colors are all there. No light other than the Sun's is used and no photographic tricks or special materials were implemented.

RJD in Dallas wrote:Instead of sunlight, the source of light is the camera flash.

I don't see how it could be the flash from the camera. If the flash went off on the camera he's holding in front of him, how could his shadow show up? The shadow needs light in back of it for us to see it.

I think it's a glory.

Assuming the ring of light is a glory of sunlight behind the observer/photographer, I would guess that the bright area in the ring is due to refelection on the outside of the crystals/droptlets, whereas the glory, like a rainbow is light that is reflected off the far side of the crystals/droplets and refracted, hence the colour spectrum.

I once saw a halo around the moon which was about 15 degrees radius, many times the moon's apparent size, which I believe (!) is caused by droplets of water that are larger than the normal fine mist that we're used to seeing in front of the moon. My question is whether the angular size of the glory gives us the size of the crystals/droplets, and whether a wider rainbow would imply a greater rafractive index. In other words, can you determine the exact nature of the small refractive and reflecting particles from a photo like this?

I have no idea about the cause of the rings but the surface it's reflecting off of looks like a layer of ice on top of asphalt. It also seems that the person creating the shadow is wearing a heavy jacket of some sort.

Just my 2 cents,
TJ

I have no idea about the cause of the rings but the surface it's reflecting off of looks like a layer of ice on top of asphalt. It also seems that the person creating the shadow is wearing a heavy jacket of some sort.

Just my 2 cents,
TJ

My guess is that he took this in a parking lot with a light oil film somewhere between the tropics of Capricorn and Cancer on a date/time when the sun was directly overhead. The ring is the diffraction pattern of the oil, which is circular because the sun is directly over his head.

the circular patter is due to the reflection of the sun (from the white matter on the pavement - the one used in marking lanes) into the camera lens and then back to the pavement.