Strobe lights can make rotation appear to "freeze" when the frequency of the light matches the rotational rate.
A strobe light is a light that flashes off and on at a very quick, regular rate. When other light sources are turned off, objects are invisible when the strobe is off, and can only be seen when the strobe is on. This can create some interesting effects when objects are moving, as they appear to jump from place to place. Because of those interesting effects, strobe lights make excellent laboratory tools.
Terms
A strobe light makes continuous motion appear to be separated into discrete steps.
Strobe lights have a variable frequency. That means the rate at which they turn off and on can be changed. Frequency refers to how many times the light turns on per second, and it's quoted in Hertz, which is simply cycles per second. So a light that turns off and on 30 times per second is operating at 30 Hz. The time between flashes is just the inverse of the frequency. So a light with a frequency of 30 Hz takes 1/30 of a second from flash to flash. The velocity of an object can be measured by knowing the distance it moves between flashes, and can be calculated when you know that each flash is 1/30 of a second after the previous one.
General Motion
If you put a ball on a table and illuminate the table with a strobe light, you can tell things about the motion of the ball. You can take a long exposure picture of the table and --- if the ball is moving --- you'll see a group of separate images of the ball in the photograph. If the distance between images of the ball is the same for every flash-to-flash period, then there is no external force working on the ball. If the distance between images changes, that means the velocity of the ball is changing, and the ball is being acted upon by a force. This same kind of experiment can be done with any object in motion.
Rotation
If you mark a black disc with a white circle on one edge and attach that disc to a spinning motor, a strobe light can be used to measure the rotation rate. You'll have to adjust the timing of the strobe light until the white circle appears to stay in one spot. That means that the disc rotates once in the time between flashes --- or does it? It could also mean the disc rotated twice, or five times or any whole number of times between flashes. You can investigate this by changing the strobe frequency and seeing if you can make the circle "freeze" again. Some turntables use strobe lights to monitor the speed of a disc's rotation and make the sound reproduction more accurate.
Vibration
One of the most interesting applications of strobe lights is with vibrating objects. Piano strings, drums, wineglasses --- all change their shapes when they vibrate. They vibrate so quickly that your eye cannot see the shapes they take on. But when a strobe light is adjusted to a frequency that matches the vibrational frequency of the object, the motion "freezes" in the same way as the rotating disc. Also in the same way as the rotating disc, the frequency of the vibration could be exactly the same as the strobe light, or 10 times, or 136 times, or any whole number of times greater than the strobe light frequency.
Tags: strobe light, between flashes, strobe light, when strobe, between images, distance between, distance between images