Lewin's Challenge Image

[Cal Harth]

JohnD, I read Emma's post on page five and knew immediately that she knew how the optical effect was produced. I don't know what bow glass is, but I would be surprised to see it coming out of a sandblaster's nozzle. Lots of good thinking went into our collective responses. I hope this continues.

[mike.roman@comcast.net]

It appears to be the reflection on the ground of the sun shining onto the lense of a camera. The rings are from the distortion of the lense at the edge of the metal ring of the camera.

[-=Arramon=-]

Wow.. that's very interesting.. again. I luv this place... wish I could get registered.. (not working btw.)

So what if you sky-dived and there was a rainbow in the sky? Would you continually see primary and secondary rainbows scroll up as you fall down?
(Or however the effect may be)

Just curious..

=)-~

[Dan - Mt. Shasta, Ca]

Dont be so hard on Emma..she didnt spill the beans..the good Professor told everyone they were "correct"..smart guy. Otherwise he'd get 500 emails a day.

[coloclimber]

Simple. It's the flash from his camera reflecting off of the mineral content of the asphalt. As the saying goes, In New York (And Massachusets, evidently) the streets are paved with diamonds. The refraction causes us to see the rainbow as we are used to.

[RR]

I see lots of light but no dark. A light source?
I also see finger rubs not foot prints, a wall?
The circle is perfect and the color bands in the rainbow are very close in size.
Not a natural occurance I'd guess.
Yes it looks cold, june in Mass. Ice. water droplets. can we get the right angle to make a real rainbow?

I think someone stayed up all night looking at stars and when the sun came up they pointed their telescope at it and projected the light onto a wall. the rainbow is from difraction of the suns rays through some morning mist.

Rockin Roy

[rasa]

Isn't this called an anti-solar halo? It's a natural occurance but, unfortunately, I don't know how it happens.

[analogee]

hell if i know.

[Titon]

Lewin is inside a camera obscura. That's why you see a bright spot (the sun's image) against a darker background (otherwise, everything would be equally bright). This is maybe because the construction site is a closed one, and there's a hole somewhere in the ceiling allowing a sunbeam to enter.

Now, this bright spot (corresponding to the sun's shape) has raibow colors around. That is because of difraction, which is because of different speeds of light for different densities of a transparent means, most probably light going through air and water. My guess is that droplets of water were inside the camera obscura. Somewhere inside the closed construction site, water was being sprayed.

So, my guess is that this is a rainbow for a single beam of light coming into the dark construction site and caused by droplets of water being suspended in the air and through which the beam passed.

[Guest]

He does not need to be in a camera obscura, note that the outside of a rainbow is usually darker than the inside. Forget the dark place with a hole on the top...

Lewin is inside a camera obscura. That's why you see a bright spot (the sun's image) against a darker background (otherwise, everything would be equally bright). This is maybe because the construction site is a closed one, and there's a hole somewhere in the ceiling allowing a sunbeam to enter.

Now, this bright spot (corresponding to the sun's shape) has raibow colors around. That is because of difraction, which is because of different speeds of light for different densities of a transparent means, most probably light going through air and water. My guess is that droplets of water were inside the camera obscura. Somewhere inside the closed construction site, water was being sprayed.

So, my guess is that this is a rainbow for a single beam of light coming into the dark construction site and caused by droplets of water being suspended in the air and through which the beam passed.

[RickyC]

There are two things in the photo that need explanation. The first is that the Professor's shadow casts a sharp image i.e. one produced by a point light source. It would not be produced by having anything between the Sun and the surface on which the shadow is projected. So the photo has at least the Sun as one light source. The shadow also shows an 'old-fashioned' type camera with the large flash holder sitting on top of the camera. The 'halo' effect is centered on this flash unit, and would then indicate that the bright circle and the 'Newtons Rings' at the periphery have been produced by the camera flash unit. This 'bright ring' and 'Newtons Rings' are standard phenomena for sunlight being diffracted through very thin translucent materials (usually ice in the upper atmosphere, as some of the other readers have suggested.)
However, there seems to be a contradiction here! We have one photo taken during daylight hours (the shadow) and another taken when only the flash will produce the interference rings. So a postulated solution - this is a double exposure photo with the first exposure taken in the day, while the second exposure was taken at night with a very thin layer of ice on the surface. Am I correct? (Careful observation of the shadow of the Professor's clothing at the bottom of the photo shows that he is wearing something like a large coat - something which could also hide the shadow of the object (tripod?) on which the camera is mounted.)

[mad dog md]

is the professor, photographing his own reflection, using light reflected off window glazing (possibly even a wall of glazing, such as most large buildings these days exhibit), onto new age ceramic ground cover. probably new construction or addition work. probably midlevel mist, though ambient water vapor may be enough. explains inverted rainbow color pattern.
/lurker
//harvard '77

[JohnD]

All,
I fear that Prof Lewin has a very low opinion of APOD viewers, if he allows his friends(?) or students spoil the fun, as if they couldn't do so without help or thought more than a week would be needed to penetrate the answer.

No, Boo! to the Prof (and Emma) and Hooray! for APOD!
John

[Dave]

It appears that the sunlight is being refracted from the fine white powder on the surface of the concrete. Whether this is sand or sheetrock dust, I can't tell. White sand would be my first guess. The Professer's person is acting like a chronosphere, blocking out the body of the sun behind him, and allowing the light rays to bend around him to form the white portion of the circle. Taking the picture on the solstice when the sun is highest in the northern sky accounts for the almost perfect circular ring.

[smithpw]

1. The rainbow ring is caused by light being refracted through the material that is seen scattered on the ground -- this appears to be the glass spheres that are mixed in paint to make it reflect light (like that used in paint on highway lines). Since I'm not certain of what the material is, I cannot be more specific.

2. Another explanation is due to the refraction of light that has passed through a lense (photographer's glasses or a sighting lense of the camera used to take the picture).

3. The bright circle of light may be casued by light passing through an optical lens (either glasses of the photographer or an exposed sight lense on the camera used to take the picture. The source of light is directly behind the photographer as is apparent by the shaddow.

smithpw

[Federico]

It can't be the flash from the camera, because you can see the photografer shadow, the light provides from the Sun, but it's the "mineral content" of the asfalt (little spherical-shaped bits of glass) the result of the rainbow we can see, and it's the result of the brighter zone inside the rainbow to the outside the rainbow zone.
Inside each glass sphera the light is refracted when gets in and then it's reflected two or three times and it's refracted again when it get out the glass, so it's the refractation of the light when it gets inside and outside the glass the result of the rainbow.

Simple. It's the flash from his camera reflecting off of the mineral content of the asphalt. As the saying goes, In New York (And Massachusets, evidently) the streets are paved with diamonds. The refraction causes us to see the rainbow as we are used to.

[HAMASAKI Syuu]

In the picture, I can see many small glass beads are spreaded on the street. they work as rain dorps on rainbow.

If transparent beads are illuminated by the Sun, we can see a rainbow.

[milman]

The same sort of thing seems to happen when on an airplane. I was looking out my window, and there seemed to have been a "spot" or "area" that appeared brighter than anywhere else on the clouds below(it's really hard to explain. Mind you, this is at 30,000 feet.) I looked across the isle through the other window, and the sun seemed to be right at about the position opposite from the "spot"; as if a straight line was going from the sun, through the plane, and to the "spot" on the clouds. Almost an inverse shadow.

Is this similar?

[twocents_n_ga1]

its the sun shining through a magnifing glass

[dangeisler]

this must be a little like the moon's halo. I have seen it during an approximately half moon where the shape of the halo is like a mirror image of the moon--the straight side of the halo is on the right while the straight side of the moon is on the left.

And I do know that the moon doesn't really have a "straight side," so no jokes.

[spoutnicko]

first of all, it seems very similar to a rainbow, with its inner part brighter, but it is obviously not one: firstly, the colored part is too much wide in comparision with its total diameter, and there is no secondary bow. then I thought it could be a special kind of glory but this phenomenon is not caused by a diffraction effect because diffraction is proportionnal to the wavelength. it looks also like a kind of heiligenshein but the colors prove it is not.

then, the analogy with a rainbow makes me think that it is refraction and dispersion taht cause this effect, and thatit could be a bow but not made with water, with a part of heiligenshein effect.

then I tried to find out what was the angular size of the bow. basing on the small stenope effect in your shadow and knowing the angular size of the sun, and then basing on the perspective effect on the ground and your head size, I estimed it measured between 35 and 65 degrees so it is much smaller than a rainbow.
so it seems that the refractive index is around 1.45 which is quite near the one of glass (a glass box should have a diameter of 43degrees. the fact that the colored part is wide show that the constringence is more important that the one of water, which is the case with glass.

the bright inner part should so in my mind be an heiligenshein effect due to reflection on the superior part of small glass balls.

so I think this strage effect could be caused by small balls of glass (or another material with an index near 1.4-1.5) on the groud

others possibilities should be bows of another material, for example with dropplets of a liquid sprayed in the air, with also an heiligenshein effect

nicolas L.

[direland]

Hello Dr Lewin

My guess about your Astronomy Picture of the day is that it is a variation of a Heiligenschein (not a glory) and that in this case it is caused by mineral crystals that act like tiny corner cube prism reflectors (like the Apollo ones on the
Moon) . I think the color sequence is caused by the number of internal reflections being an odd number (3) so the most refracted short wavelength blue light is inside the red. The diameter of the circle would be a function of the index of refraction of the mineral crystals.

Dale Ireland
direland@drdale.com
Live webcam http://www.drdale.com/cam

[Ramaus]

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.

[Marcel C. (NL)]

This phenomenon is known as the Brockenspectre. It can be seen when looking exactly at the anti-solar point under certain conditions. (i.e. refracting materials like moist on the ground)

[alg]

Dark interior shot of dirty, wet (condensation), reflective mirror glass with light behind shooter probably from interior security light.