Lewin's Challenge Image

douglasj · Post by **douglasj** » Wed Sep 15, 2004 6:58 pm

A circular projecting white light source or one projected through a circular aperature onto a surface with lower lighting level. The subject photogarapher stands between the light source and surface. This explains the photographers shadow in a circular lighted area projected onto a surface of lower lighting.

The surface is asphalt with a light layer of silica sand spilled or sprinkled on it. The silica sand, being prismatic in shape and fairly clear, through internal reflection creates the rainbow effect.

Marc Valdez · Post by **Marc Valdez** » Wed Sep 15, 2004 9:20 pm

Some sort of heiligenschein. The bright central circle is caused by retrodirection of sunlight on spherical droplets. The color pattern is caused by refraction of sunlight and (one) internal reflection within the droplets, with shorter wavelengths bent more than longer (hence with blue on the inner periphery of the ring). What is unclear is what kind of droplets these are: water or glass. Water's index of refraction is lower than glass's, which means water's heiligenschein will be smaller than glass's. I'll guess a dew heiligenschein.

Ciara · Post by **Ciara** » Wed Sep 15, 2004 9:26 pm

AdiabaticDannis wrote: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

No offense, but, I think that your quote, although imaginative and interesting, makes no sense at all and it is totally false. Not to be rude at ALL!

j-def · Post by **j-def** » Wed Sep 15, 2004 9:49 pm

For those commenting that the dispered colors are indeed a "glory", why has nobody commented on the particle SIZE, which is irrelevant in rainbows, but completely deterministic in glory formation.

Also, am I the only one to notice Lewin's bike helmet?

King Ables · Post by **King Ables** » Wed Sep 15, 2004 10:31 pm

I don't think it has anything to do with what's on the surface.

I've seen this exact same phenomenon from an airplane and
always wondered what caused it. Looking down at the surface
from my window (at cruising altitude) I saw this ring moving
with the plane across varied terrain. The plane shadow was in
the center of the ring, so I assumed it had something to do
with my relative position in the "cylinder" of light between me,
the sun, and the ground, that caused some strange refraction
right around me.

Interestingly, the relative size of the ring was about the same as
in this picture, although in my case, if it was on the ground, it
would have been miles long... so I suspect the cause of the ring
was close to me as opposed to close to the ground.

But I don't have an explanation, only another data point.

-king

Brian (Carpenter) · Post by **Brian (Carpenter)** » Wed Sep 15, 2004 10:34 pm

Very shortly after the pouring of this concrete slab, in the heat of the day while the sun is directly overhead and while the concrete accelerates in its cure, the concrete heats up and along with the additional heat generated by the sun the concrete very rapidly expels its moisture content. Setting the camera to use the flash feature, the professor takes his shot, he is strategically positioned at 90 degrees above the wet surface of concrete. With a sudden blast of light generated by the cameras flash and with the surface expelling large amounts of moisture into the air a very simple experience occurs. That very same principle that we all see as a common rainbow. The only difference is that the light is corralled into the center of an area of moist air. Being 90 degrees above this damp hot air causes the ring to occur as the light struggles to deflect outwards. The simple prismatic effect of the water molecule finishes the image. The center white light blasting its way outward through the moist air. The colors? That of a rainbow. Now try and spot the gold!

NotAGlory · Post by **NotAGlory** » Thu Sep 16, 2004 6:51 am

This was posted by "Emma" it can be found on page 5, it has been confirmed by Dr.Levin

This is a the same as the physics occuring when a rainbow is formed. The ground must have had some kind of grains in it that were spherical in shape which light can pass through.

Consider just the top of the rainbow pattern. The light refracts once upon entering the grain and the blue light is refracted through a greater angle than the red light as it has a larger index of refraction. The seperated light then reflects of the back of grain and the blue light is therefore deflected upwards more than the red. The light is the refracted again upon leaving the grain causing an even greater seperation. Because the blue light is upward more, the blue light which reaches the camera comes from grains which are lower down. This happens the entire way around in a circle and explains the colour ordering.

The bright area in the center is also a phenomenon which can be seen in rainbows (there is bright section either side of the primary and secondary rainbow with a dark patch in the center). The light paths for parallel rays through the grain which produce the rainbow pattern come from the grains that are furthest from the center of the circle which have single reflections. All others reach the eye from further in grain which lighten the area. This effect is made even more noticable because any grains further out than the circle can not bend the light towards the camera which makes this region darker than the actual ground.

There should be a 2nd circular rainbow further out on than the photo with the reverse colour order which corresponds to light which is doubly reflected within the grain. Further out from here would be another light patch.

A good website which explains rainbow physics is

http://hyperphysics.phy-astr.gsu.edu/hb ... on.html#c1

Henk de Zeeuw · Post by **Henk de Zeeuw** » Thu Sep 16, 2004 11:20 am

Prof Lewin is looking to a wall. Someone near him is holding a CD.
The sun is shining on the CD
The middle light part is the reflection of the CD.
The colors are inteference colors.
I saw the same picture on the wall in my room when a CD was laying on the table in the sun

Jeremy Australia · Post by **Jeremy Australia** » Thu Sep 16, 2004 11:35 am

I know for a fact that Emma (on Page 5) was absolutely SPOT ON with her hypothesis. I too emailed Prof Lewin and he confirmed that I was right (in a very round-about kind of way) and Emma explained this phenomenon correctly.

rms · Post by **rms** » Thu Sep 16, 2004 11:47 am

I think mahndrsn is close, but the refraction derives from moisture interacting with the asphaltic oils exuded by the pavement. The sheen is similar to that observed in gasoline and/or oil.

BuckDharma · Post by **BuckDharma** » Thu Sep 16, 2004 6:40 pm

Can't you see that its Jesus returning to punish the sinners?

Patrick · Post by **Patrick** » Thu Sep 16, 2004 7:23 pm

You know on construction sites where they spray from water trucks to keep the dust down? I'm thinking it has to do with that or condensate from water previously applied. When do I get into MIT?

alevand · Post by **alevand** » Thu Sep 16, 2004 7:27 pm

Get back to work, Adam.

Greg P · Post by **Greg P** » Thu Sep 16, 2004 8:29 pm

I'm going to suggest this is coming from the camera itself, with the sun shining through the viewfinder such as on a rangefinder camera.

Kafi Benz · Post by **Kafi Benz** » Thu Sep 16, 2004 11:03 pm

My response became posted by itself -- as a new topic -- hope all are examined.

Squale · Post by **Squale** » Fri Sep 17, 2004 10:23 pm

Hi all...

As to me, the answer is that the feature you can see on the picture is a "glassbow". It has the same principle as a classical rainbow, except that instead of reflecting and refracting the sunlight in water spheres (drops), reflection and refraction are made in the glass spheres used on the building area.
The phenomenon has nothing to do with glories or coronas, which are produced by diffraction of the light, and are ALWAYS brighter near the antisolar point (the shadow of the observer), and dim as you look further away. Here, you can clearly see the rainbow-like phenomenon, whith a sharp red edge on the "outside", the specter colors, and some interferences, and finally, the inside glow produced by reflected light.
Nice picture anyway. I would have liked to have some details, for example about the lens that was used (focal length), the camera type, etc... This can be useful information to analyse such optical phenomena.
Regards,
Squale

Ramaus · Post by **Ramaus** » Sat Sep 18, 2004 1:39 am

It's a normal simple rainbow. The lighter area inside the circle, and the blue on the inside of the circle and red on the outside confirm it. The water droplets are between the camera and the darker background scene, probable from a lawn or flower mister. You could do the same thing with a lawn sprinkler.

slangill@dcaccess.net · Post by **slangill@dcaccess.net** » Sat Sep 18, 2004 3:59 pm

The phenomena is a corona of interefernce colors (in the same color
sequence as Newton's Rings). The corona is seen on wetted oily ground,
on wetted road asphalt (ie with auto and other oils), or on ferruginous
soils (caused by a colloidal solution of iron oxide). The bright spot is
due to the camera's flash.

NW9784 · Post by **NW9784** » Sun Sep 19, 2004 7:33 pm

"The Glory" reflecting directly opposite a source of light. Small spheres reflect/refract/diffract light (in this case the sun) back to the light source with the observer (cameraman) becoming the silhouette in the center of "the gegenschein" as the picture is taken of the relflected halo.

Norman

ICE · Post by **ICE** » Mon Sep 20, 2004 1:42 am

I happen to know this topic at a japanese BBS. On the BBS, they said that this phenomenon is known very well as the experiment of rainbow beads. So, I'd like to introduce some pages that present the experiment of rainbow beads. The middle of the following page you can see school children making rainbow screens that make same optical phenomenon.

http://www2.hamajima.co.jp/~tenjin/album/study/sky.htm

The rainbow bead is able to obtain from Nakamura-rika ( http://www.rika.com/jikken/jikken201406.htm, support@rika.com). I'm not sure they accept foreign order or not.

beep beep · Post by **beep beep** » Mon Sep 20, 2004 3:58 am

Well the refraction of light is originating from the camera flash and reflecting off some phosphorescent material ... perhaps the aforementioned "highway beads" , or various mineral dusts deposited from the nearby constuction site ........ How's that ???

Bob Peterson · Post by **Bob Peterson** » Mon Sep 20, 2004 2:15 pm

The Prof sees the Complete Ring of Colors and the Brighter Area(scattered light producing white light) within the refractive arrangement of light (the Rainbow Ring). On the ground are glass beads with the same refractive characteristics of the rainbow beads seen on the LINK below. The Sun is high in the sky, but behind the Prof, and is the source of light creating the phenomenon. The Prof's shadow was cast DIRECTLY onto this phenomenon; It's not the BROCKEN SPECTRE. So, in conclusion, the Prof sees the phenomenon on the ground created by the Sun acting on scattered glass beads, capable of refracting light into the color arrangement of a primary rainbow and a bright area of scattered light. (the Japanese bead link was very compelling). The Prof with the SUN to his back, (thus casting HIS OWN SHADOW DIRECTLY onto the phenomenon), takes his photo. Credit to the beadheads. Credit ICE for this Link: http://www2.hamajima.co.jp/~tenjin/album/study/sky.htm

MadMax · Post by **MadMax** » Mon Sep 20, 2004 5:11 pm

It's obviously an MIT prank carried out by juniors trying
to make more bald headed professors.

mart · Post by **mart** » Mon Sep 20, 2004 8:07 pm

it looks like sand spilled on the ground and the light reflecting from behind the camera man is causing a prizmatic effect.

Guest · Post by **Guest** » Mon Sep 20, 2004 11:31 pm

could this be a demonstration of the speed of light caught in a moment of time?

it looks to me like this photo is taken at night, on damp pavement. if it's completely dark, there would be no "previous" reflection from the pavement. if the light is bright enough, and the shutterspeed quick enough, and the timing is just right, in theory you could capture the light beginning to luminate the surface area. this could also explain the "reverse" rainbow, but that could also be a lens fair.

intriguing stuff!

elija

Starship Asterisk*