Photograph courtesy of Bunnie Huang

Flash Dancing

Why do we perceive a path of light when a dancer moves flashing lights in the dark? That’s “ persistence of vision.” Our eyes and brain continue to see a light for 15 to 30 milliseconds (ms) after it has gone out. An image that updates once every 15 ms updates at a rate of 66Hz (frequency is the reciprocal of period). It’s no mistake that AC power uses 60Hz to eliminate any flicker from light bulbs.

You may also notice this effect when setting your monitor’s refresh rate. Another example is the Fantazein™ clock; it employs a bar of LEDs swinging on an inverted pendulum to trace out digits that appear to float mysteriously in thin air.

Figure 1: A dancer swing-
ing a kinetic glowstick
on each arm creates two
different patterns
of light.

Make: 35