Strange occurrence on office wall.

[Jilang]

Quite late this afternoon I noticed a series of perfectly formed circles of light on the wall above my computer. Where are they coming from I wondered? Putting my hand in front and tracing one back the window, then climbing onto a chair it was clear they originated from the little holes at the top of the vertical window blinds. What was weird though is that the holes are perfect squares, but the light patterns were perfect circles! I invited my colleagues into witness this "strange phenomenon" and they went away scratching their heads. Wanted to share this as it was the most entertaining thing that happened today at work!

 

[mfb]

The sun is a nearly perfect circle, and the small hole is nearly like a point (compare the spot size to the hole size to check this), so you have a cone of light coming in and a cone of light going out. Basically a pinhole camera image of the sun.

During solar eclipses, all those spots become crescents and you get fascinating images like this.

 

[Jilang]

Spoiler! LOL.

 

[jtbell]

If the sun were a point object, then the square apertures would indeed produce bright square spots on the wall, except for possible diffraction fringes.

The sun isn't a point object, so it behaves as a collection of point objects, distributed over a circular disk. Each of these sources produces a bright square spot on the wall. The centers of these square spots are distributed in a circular disk, so the square spots overlap to produce a larger circular disk.

 

[Jilang]

Thanks folks. It did have me scratching my head for a minute too until I saw some clouds going past over the disc. Couldn't see any sunspots though!

 

[sophiecentaur]

You just happened to make yourself a pinhole camera!

A few years ago, the Sun was in just the right position to shine through a small hole in the back bedroom window blind in my house and form an image on the front door. via the staircase, at just the right angle. The 'throw' from hole to door was long enough to produce an image of a couple of cm diameter. You could clearly see some sunspots on the image. My son and I were 'well impressed'.

 

[Cephas Atheos]

I saw some wonderful curves from sunlight scattered from a half-full glass of light machine oil. Bernoulli curves, cycloids, and combinations kept a colleague and I entranced for nearly an hour!

We even tried adding water to see more - and different coloured - curves.

I was still gobsmacked by the tree shadow image from mfb's link. Wonderful.

 

[davenn]

Thanks folks. It did have me scratching my head for a minute too until I saw some clouds going past over the disc. Couldn't see any sunspots though!

that's a pity as there's a couple of big spots on there at the moment

Dave

 

[Jilang]

that's a pity as there's a couple of big spots on there at the moment

Dave

I guess the resolution was not good enough, the holes are 4mm square. If I see it again I have a plan involving Blue Tack...

 

[A.T.]

What was weird though is that the holes are perfect squares, but the light patterns were perfect circles!

Measuring from the wall, the holes have smaller angular size than the sun. If you increase their size, or bring them closer to the wall, the spots will become squares.

 

[sophiecentaur]

I saw some wonderful curves from sunlight scattered from a half-full glass of light machine oil. Bernoulli curves, cycloids, and combinations kept a colleague and I entranced for nearly an hour!

We even tried adding water to see more - and different coloured - curves.

I was still gobsmacked by the tree shadow image from mfb's link. Wonderful.

I'll have some of what he's having.

 

[CWatters]

I made a crude pin hole camera to observe the transit of venus a few years ago. It worked quite well. Why not try it yourself next time ;-)

 

[Jilang]

Thanks CW, I'm guessing the sun needs to be out right? This is a rare occurrence at the weekend so it looks like it will need to be another office experiment!

 

[mfb]

I made a crude pin hole camera to observe the transit of venus a few years ago. It worked quite well. Why not try it yourself next time ;-)

Like... December 2117? ;)

I'm guessing the sun needs to be out right?

The contrast is orders of magnitude better with a visible sun, right.

 

[Jilang]

I'm worried I'm going to miss it! Just trying to calculate the odds of it being daytime, sunny and occurring in my lunch break...would I stand a better chance in Hawaii?