Sorry for the long delay in posting (not that it matters — I see my stats — most of you are off reading my old posts about how water goes around drains or whether polar bear fur is fiber optic). I’ve been on vacation back in my old haunts in the SF Bay Area, and thought that I would have lots of time to post, but I was too busy enjoying myself.

While I was back, I stopped through my old alma mater — the Exploratorium — and watched Paul Doherty teaching about light. You might know the old trick of using a diffraction grating to see the rainbow. You put a diffraction grating over a bright light (an overhead projector works great) and you see white light projected on the wall. Next you block off most of the light except for a narrow slit (you can cut a manilla folder to do this). You’ll see a rainbow (blue, green, red) projected on the wall on either side of the slit. What’s going on? Light bends around the tiny slits in the diffraction grating (red bends more), making infinite numbers of overlapping rainbows, which we see as white light. The slit blocks out all the rainbows that are there except for one, so we can see a clear blue/green/red pattern. (Think about that a moment, it’s a subtle point, and important).

The tiny grooves in a CD act like a diffraction grating too, that’s why they look rainbow colored.

However, it gets really interesting. Now, take away the “slit” so we just see white light again. Put an “antislit” in front of the grating… basically, a long thin strip of paper the same size and shape as the “slit” was. Instead of letting in a narrow strip of light, we’re blocking all but a narrow strip of light.

Instead of a rainbow to either side of the antislit, we now see the *complement* of the rainbow — yellow, magenta, cyan. Why is *that*?

Think about it.

WIthout the “antislit” there, you have white light, an infinite number of overlapping rainbows.

When you put the antislit there, you have blocked a “slit” — blocking the rainbow pattern that you saw with the slit there.

So what you see is “white minus blue” which is yellow, plus “white minus green” which is magenta, and “white minus red” which is cyan.

This is similar to Bob Mlller’s wonderful light walk, in which white light outside is made of an infinite number of images of the sun. When we look at the light projected through a pinhole (even if it’s not round) we see a round image — one image of the sun. If we look at the light that goes around an anti-pinhole (like a piece of paper, even triangular) you see a round shadow… the opposite of an image of the sun!

Here is the antislit activity from Paul Doherty’s website. As he puts it, “The anti-slit removes one wavelength at a time from white light. Thus we see the spectrum of subtractive colors”