How to play Strobe Freeze
- Choose a test and read the wave's frequency, the flash rate, or the creep on the card.
- One flash per cycle freezes the motion; otherwise it seems to move at f_seen = f − f_s.
- Type your prediction and press Start. The strobe flashes in real time; when the flash rate or the wheel's speed is asked, yours is used.
- Watch whether the picture stands still or creeps, and compare the orange marks with the white ones.
The strobe effect
A stroboscope is a lamp that flashes for a tiny instant at a steady rate. In the dark you see a moving object only at those instants, and your eye blends the images. If something repeats its motion f times a second and the lamp flashes f times a second too, every flash catches it at the same point of its cycle, so it seems to stand still. Flash at half the rate and it still looks frozen, because it is caught every other cycle; flash at twice the rate and you see two images at once. If the motion is a little faster than the flashes, each flash catches it slightly further on and it seems to creep forward slowly, at f_seen = f − f_s; a little slower, and it creeps backwards. This is how mechanics measure the speed of engines and fans without touching them, and why wagon wheels in films can seem to turn slowly or backwards: the camera takes pictures at a steady rate, like flashes. A wheel with n identical spokes looks still whenever each spoke moves exactly to another's place between flashes.
A focused learning model
Each flash is instantaneous, and the eye holds each image for 0.1 s, fading; the real motion is shown as a gray blur. The wave is a perfect sine and the wheel has 8 identical spokes. Runs play in real time, so your screen's refresh can add a little flicker of its own. When the flash rate or the wheel's speed is asked, your answer is used. Answers within 2% are right.