Ask Mr. Science
page 18

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Why do wheels sometimes look like they are going backwards?

You can see this sometimes on TV or in a movie, and it is known as the 'wagon wheel effect' or the 'wagon wheel illusion'. This illusion can happen anytime you have two ingredients: 1) a stroboscopic (flickering) light source, illuminating or viewing 2) a moving periodic pattern. 1) can be a movie or TV camera, a stroboscopic light, or a vibrating mirror. 2) can be the spokes in a wheel, a picket fence etc.

To show the effect, I had a bicycle wheel mounted on a wooden stand, and I borrowed a strobe light (the black box on the right). I put bits of masking tape on the rim, since the spokes are not all that visible. When you turn the wheel in normal light, nothing odd is visible, but of course when you turn on the strobe, you can make the rim appear to go forward, backward, stand still etc. Check out the links below for explanations - I don't need to repeat that. On a paper disk I drew unevenly spaced spokes. Even when illuminated with the strobe, these never 'misbehave'. I've heard that in the old days, some moviemakers fabricated wagon wheels with unevenly spaced spokes in order to avoid the illusion of backward-going wheels.

Not everyone has a strobe light around the house, but if you drive the kind of car that rattles enough to make the mirrors vibrate, look in the mirror at other car's wheels, and you can see the wagon wheel illusion in broad daylight. [This is of course meant for the passengers - drivers should keep their eyes on the road].

The other thing you can see in the photo is a green rubber band. If you twang hte rubber band and adjust the strobe just right, you can make it seem as if the rubber band is vibrating very slowly - another demonstration of the illusion.

21 January 2004


How does the sun shine?

Time to talk about nuclear fusion: 1) How atoms are put together, 2) how nuclei are put together, 3) how in a fusion reaction, a tiny bit of mass is converted to an enormous amount of energy, via E=mc2, and 4) why it is so hard to get nuclei together (electrostatic repulsion), and 5) how you can do it: very high temperatures.

29 January 2004


Functions of the skin

[keep warm/cool, keep outside out, keep from drying, heat/cold/touch/pain/itch sensors, symbiosis with bacteria. Who has skin (endo/exo skeletons, terrestrial/aquatic), who has fur (heating/cooling). Functions of pigmentation, vit D, general benefits of sunlight.
January 2014


How do they get so much information on a DVD

I took the opportunity to do a bit of history of recording, starting with Thomas Edison's cylinder phonograph of 1877, which held about 2 minute's worth of sound. Next were 78 RPM records (very early 1900's) with about 6 minutes per side. I brought in some 78's, so the kids could feel their weight, and because you can actually see the sound waves in the grooves. I played the record just with a sheet of paper and a straightpin. Next I played a 33 rpm LP, also with the paper-and-pin. These hold 23 minutes of music per side. With a magnifying glass you can still see the sound waves in the grooves.

Next: CD's developed in the 1970's. Even with a magnifying glass you really can't see what's on there. I explained how you go from (analog) waves, via sampling (measuring the amplitude at regulat times) to a list of numbers, and then from numbers to bits (I skipped over compression). Bits are bumps on the CD surface. How small are those bumps? My computer tells me that an empty CD can hold 70 Mb, which is 560 000 000 bits. The recordable area of a CD is about 85 square cm, so that each bit has to fit in an area of 4 micrometers on a side. To set the scale, the width of a human hair is about 100 micrometers. Finally, a DVD can hold 65 times more than that (4.7 Gbytes).

26 February 2004

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