Computer Stroboscope

Flash dance


A desktop computer with a Cathode Ray Tube, CRT, display.

An assortment of rubber bands

To Do and Notice

Turn on the computer and make the screen as white as possible.

Hold the rubber band in front of the computer screen. Stretch it vertically.

Pluck the rubber band and watch the waves appear to move slowly on the band.

Experiment with different tensions, lengths, and weights of rubber bands to find the best effects.

Wiggle your fingers in front of the screen. Notice how they appear.

What's Going On?

The image on a the cathode ray tube display of a computer screen is drawn by a dot of electrons which excite phosphors on the screen. The dot travels left to right and top to bottom across the screen. On television sets the dot travels top to bottom every 1/60 of a second in North America. The dot on computer monitors may have different times such as 1/72 of a second. These slightly faster scan rates reduce human perception of flicker. The dot travels left to right so quickly, in under 1/15,000 of a second, that we often treat the light as an entire line called a "scan line." These scan lines move down the screen.

The rubber band oscillates when you pluck it. These oscillations are usually too fast to see. When you pluck the rubber band waves travel up and down the rubber band. The travelling dot of light on the screen strobes the waves on the rubber band. Thus you see the waves oscilate in slow motion when the waves on the rubber band are travelling vertically.

Hold the rubber band horizontally and you will not see the waves travel along it.

The scan lines move down the screen and are blocked by different parts of your wiggling fingers at different times. The stroboscopic finger shadows you see on the screen thus look curved.


These activities come from the Ontario Science Center.

Look at the computer monitor through the blades of a spinning fan. Turn the fan on and off and watch the effects as the blades speed up and slow down.

Hold a pencil vertically in front of the screen. Sweep the vertical pencil quickly to the right across the screen. Hold it 25 cm in front of your eyes. Notice that you see a diagonal stripe on the screen as the moving pencil blocks the light from the screen. If the pencil starts at the left side of the screen then the diagonal line should slope down to the right. The scan of the screen starts at the top and moves down, the pencil moves right, the combination produces a line that is down and to the right.

Hold a pencil vertically in front of the screen. Hold it near the bottom and sweep the top of the pencil rapidly back and forth like the blade on an automotive windshield wiper. Notice that the pencil appears bent.


Return to Activities

Scientific Explorations with Paul Doherty

© 1999

7 May 99