Educational Innovations, Inc.

Mysterious Glowing Ball
Item OPT-500/505

From a single glowing ball, one can observe white light or alternating red, blue, and green light.

Activities

In a darkened room with the ball turned on:

  1. Hold the ball stationary and observe white light.  Move the ball a few centimeters quickly back and forth and observe the alternating slivers of red, blue and green light.
  2. Hold the end of the string and spin the ball in a large circle.  Observe the alternating colors with dark spaces in-between.
  3. In front of the classroom, remove the string and toss the ball back and forth to someone several meters away.  Students in the back of the classroom observe a white ball being tossed. However, if these students stare at the wall, instead of allowing their eyes to follow the tossed ball, they will see an alternating red, blue and green ball being tossed.
  4. Juggle three mysterious glowing balls or use two to demonstrate Poi. This is fascinating to watch in the dark!

Explanation

Inside the ball are three colored LED’s that rapidly cycle red, blue and green light.  When the ball is stationary, the eye focuses all three colors on the same part of the retina and our brain interprets this as white light.  This is due to the persistence of vision.  The brain remembers each of the different colored lights for about 1/10 of a second.  This indicates that the color cycling of the LED’s is faster than 1/10 of a second.  If the light is focused on different parts of the retina, individual colors are seen.  When the glowing ball is tossed, the natural tendency is for the eye to follow the ball and white light is seen.  If the colored ball is tossed across the field of vision while the student is staring at the background, then colored lights are seen.

Challenge Questions

  1. What colors do you see when looking at the glowing ball through different colored filters?
  2. Relate the persistence of vision to watching a movie or TV.
  3. Can you discover a way to measure the persistence of vision time?  Is it the same for everyone?

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