Sometimes rare and wonderful things happen. This weekend, for example, it was sunny in Seattle.
Which allows me to share pictures of one of my all-time favorite Cool-Physics-Around-the-Home tricks:
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Anyone who has blinds in their home or office has probably seen something like the pictures above — sunlight streaming through cracks in the blinds, producing a row of spots on the opposite wall. It would be natural to assume that those nice round spots are due to the nice round holes that were poked through your miniblinds at the factory. However, you would be wrong. Those spots are actually pinhole camera images of the Sun!
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You observe the same effect, albeit more subtly, when sun comes through tree branches. The network of leaves and branches creates many small holes, each of which produces its own pinhole image of the Sun. These images tend to overlap, making the circles less obvious than with blinds, but you can still see the faint imprints of the circles in places. Sometimes, however, the Sun is not actually round. At sunset when it’s low on the horizon, parts of the Sun can be blocked by trees and buildings. In that case, the pinhole camera images are not round either. I took the picture above shortly before sunset, when the lower half of the Sun was blocked by a neighboring building. You should be able to see that the pinhole images have turned into half circles. (The half-circles are upside down, since a pinhole camera inverts the image.) The effect can get even weirder than the picture above. For example, there’s a large bridge due west from my office window, and sometimes the Sun sets directly behind the bridge. When the Sun is partially eclipsed by the bridge deck, I can even see pinhole images of upside-down trucks driving across the bridge when my blinds are down.
How cool is that!??!! (Ok, I admit, it’s probably not as cool as the Higgs, but it’s a hell of a lot easier to see). I was pretty old by the time I figured this out. The first time I noticed it was during graduate school in the courtyard of a building at the Institute for Advanced Study, during a partial eclipse of the Sun. The moon was blocking about 2/3 of the Sun, making a tidy little cresent. All the tree-dappled sunlight on the ground turned into cresents as well. The effect was spectacular, if not a little odd. I wound up watching the rest of the eclipse on the ground, rather than through my carefuly prepared piece of mylar (i.e., the wrapper off a Poptart). Mmmmmmmm….Poptarts!
January 29th, 2007 at 2:56 am
Julliane, at least you figured it out for youself. I came to know this only after I read a book by Minnaert which was about light and color. I felt quite stupid about not noticing such an obvious thing. Fortunately or unfortunately I was not the only one who was struck by this.
The must be something wrong with our physics education. Shouldn’t we start from familiar phenomena happening around us?
January 29th, 2007 at 3:15 am
It’s _very_ cool. And why not let yourself be a window blind? If you interlock your hands like this, then, if you are in a special place at a special time, you might see THIS. I took these snapshots in Cappadochia, Turkey last March 29 from a hilltop where I and my new friends were watching the total solar eclipse. These Turkish boys were watching what we were doing with our interlocking hands, and wanted to try it out too. What fun!
January 29th, 2007 at 6:21 am
It is a beautifull phenomena. I was embarassed (but did not show, I hope) by an engineer who told me the spot was actually the Sun. Of course!
I should know better. After all I knew the concept of the pinhole inverted image in a camera.
I wonder if it is possible, in a dark room, to see some Sun spot. I never had the chance to try. Is it possible?
January 29th, 2007 at 7:00 am
The sun does not appear round at sunset sometimes even without these buildings and such in the way. In that case, it’s an effect of refraction in the atmosphere.
January 29th, 2007 at 8:57 am
I was once walking through a courtyard at MIT, and looked down to notice thousands of little crescents flittering on the sidewalk. It was, of course, the pinhole-camera effect caused by the leaves of the trees above — but during a partial solar eclipse! Beautiful and dramatic.
January 29th, 2007 at 10:10 am
hmm. i never close my blinds. so, in your honor I just attempted to let them down since the sun is shining very nicely today up here in waterloo. the only thing that happened was that the whole thing came down, leaving three holes in the wall and dust on my desk.
well, already in college i’ve always managed to mess up every possible experiment.
very nice post btw
January 29th, 2007 at 12:56 pm
I remember seeing the same effect as Sean mentions during the solar eclipse in 1999, but on thinking about it later, I assumed I had just been imagining it. It’s nice to think I *should* have believed my eyes then
January 29th, 2007 at 3:36 pm
Somehow, I don’t think that’s so much a physical phenomenon as a geometric one. I pointed out to a professional photographer friend of mine once that when you bring the flood light closer to your subject, the shadows get less sharp, and he had trouble believing me, till he tried out for himself.
January 29th, 2007 at 9:09 pm
Thats cool, but my curtains is even cooler. They demonstrate the wave nature of light. I have some semi-transparent curtains in some synthetic textile. One time I noticed that the streetlights shining through the curtains was broken up into rectangular patterns of dots. I figured out that this can only be due to diffraction. Even though the textile is a very course diffraction grating, it could bend the light a small angle so it was noticeable.
January 29th, 2007 at 10:14 pm
One of my grad students showed me how you can get the same diffraction effect through your fingers. If you take your two thumbs and press them together side by side, there’s a little gap between the two knuckles. If you narrow the gap as much as possible while looking through them, you’ll see vertical lines, due to the same effect.
But having entire curtains do that must be cool!
January 30th, 2007 at 1:15 am
Also in Seattle, a warehouse I worked in had holes in the wall threw beautiful images of the sun on to the opposite side of the building. You can just barely see very large sunspots this way. (And just in case you’re wondering, no, I deny drilling any of the various holes. The building was previously used as an iron foundry.) Speculation is that in ancient times, the Arabs surely must have seen sunspots in their tents. And I would think that sunspots would have been clear in the images of the sun on the floors of old churches (where they keep time).
January 30th, 2007 at 7:13 am
Very nice post! I first noticed the crescent effect when doing some eclipse watching with a crowd of fifth graders during the ’99 eclipse. The effect through trees is indeed wierd and spectacular.