Interference

There is a lot of funny stuff I found in my old desk at my parents house! Among other things a 20 Mark bill... some 1000 Pesetas... a photo from me at Ipanema beach.... a truly ancient article from the Süddeutsche Zeitung (April 13th 2003) that a friend gave me, and which features Fotini Markopoulou talking about time not being continuous but quantized in Planckian units (I remember being relieved that there are people doing even weirder stuff than I, and thinking I didn't know they have cockroaches in Canada as well¹)... lots of trash... offprints of a paper I forgot I wrote... my old passport with the visa from Zimbabwe... a Miró postcard book, that is nice...

What else...? Aah, the CD from Queens of the Stone Age that I thought I lost in Tucson! And a laser pointer. This laser was extremely expensive at the time I bought it. Nowadays you get them as advertisement gifts. I exchanged the batteries and it still works.

If you have a laser pointer, try hitting a CD² with the laser... here's how it looks like:

(Click to enlarge)

Surprised?

Audio Compact Disk - An Introduction: "The CD disk is a 120 mm diameter disk of polycarbonate. The center contains a hole 15 mm in diameter. [...] The active data area starts at the 46 mm diameter location and ends at the 117 mm diameter location. The 46-50 mm range is the lead in area and the 116-117 range is the lead out area. Disks are written from the center to the outside [...]. A CD disk contains a long string of pits written helically on the disk. [...] Each pit is approximately 0.5 microns wide and 0.83 microns to 3.56 microns long. (Remember that the wavelength of green light is approximately 0.5 micron). Each track is separated from the next track by 1.6 microns.[...]"

To summarize this flash of information: the spacing between the tracks on the average CD is approximately the wavelength of visible light. That's why the CD separates the visible light into a whole spectrum and you see all the beautiful rainbows on it. The very clean and periodic spacing of tracks makes the surface of the CD act similarly as a multiple-slit, just that the light doesn't go through but is reflected back. You might have to fumble around some while before you find a good angle.

The multiple slit experiment is quite similar to the well-known double slit experiment, to which you find a brief introduction here, and an applet for a virtual experiment here. Roughly speaking, an incoming plane wave (the laser light) is split into two spherical waves centered around the slits. These interfere and cause a pattern of maxima and minima on the screen. For this to work, the distance between the slits and their width needs to have roughly the wavelength of the incoming light. The double-slit experiment was first performed by Thomas Young in the early 1800's and shows that light behaves like a wave.

The more slits there are in the experiment, the more the maxima are focused, and the less their peak intensity drops for the outside lying maxima. If you had an infinite series of slits, all the maxima would look equal. You can click yourself through various diffraction gratings on this website. The reasoning is the same if the light is reflected back: the reflection occurs in different angles, depending on how the incoming plane wave hits the tracks of the CD. The outgoing waves interfere and cause multiple maxima which you see in the photo above.

"I saw it during an optics course at Edinburgh University," wrote respondent Alison Campbell, an astronomer at St Andrews University. "The prof didn't tell us what was going to happen, and the impact was tremendous. I cannot remember the experimental details any more - I just remember the distribution of points that I suddenly saw were arranged in a diffraction pattern. Seeing the two-slit experiment is like watching a total solar eclipse for the first time: a primitive thrill passes through you and the little hairs on your arms stand up. You think this particle-wave thing is really true and the foundations of your knowledge shift and sway."

From: The most beautiful experiment

Further reading:

• Science's 10 Most Beautiful Experiments
• Young's Double Slit Experiment
• Wikepedia on the Double-Slit Experiment
• Thomas Young's Experiment
• Physics Web: The Double Slit Experiment

Ah, my husband is interfering with my blogging... his washing machine spin-dried itself towards death, so we have to go get a new one. Have a nice day!

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Footnote 1:

SZ: Was kommt der Menschheit zu Gute, wenn man die Zeit in Atome schneidet? (How does mankind profit from cutting time into atoms?)
FM: Hat es den Leuten geholfen, zu wissen, dass Materie aus Atomen besteht? (Did mankind profit from cutting matter into atoms?)
SZ: Es ist vor allem spannend. (First of all things it is exciting.)
FM: Und man hätte ohne die Erkenntnis keine Computer und 1000 andere High-Tech-Sachen erfunden. (And without that knowledge computers and 100o other high-tech-things hadn't been invented)
SZ: Keine Atombombe zum Beispiel. (And no atom bombs for example.)
FM: Auch die. (That as well.)
SZ: Haben Sie Angst vor dem Tod? (Are you afraid of death?)
FM: Nicht so sehr. Eher davor, etwas im Leben nicht versucht zu haben. Angst machen mir auch die dicken Autos hier in Nordamerika. Und Kakerlaken, die hasse ich wie die Pest. (Not so much. I am more afraid not having tried something in my life. And I am afraid of the fat cars here in North America. And cocroaches, I hate them like the plague.)

Footnote 2:

If you've all your tracks on the iPod, go ask your granny to help you out.

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TAGS: PHYSICS, SCIENCE, DOUBLE-SLIT, LASER, THOMAS YOUNG