The hand stroboscope is a disc of hardboard or card with a simple pivot at its centre, so that the disc can be kept spinning by hand.
The disc has a finger hole, off-centre, to enable the user to keep it spinning. It has narrow slits on its face, near the rim. The slits are evenly spaced and 12 slits are best. When fewer are needed, you can obscure some slits by sticking black tape over them and use 6, 4, 3, 2 slits, or just 1 slit.
This simple stroboscope enables students to 'freeze' repetitive motions – or to slow them down for closer study. For example, continuous ripples are easier to see by using a stroboscope, especially those ripples with higher frequencies. By viewing a vibrating object through the slits, students can calculate the frequency of a vibration.
The stroboscopes are less likely to judder while rotating if the bearing is not too tight and the handle is held loosely.
Photo-induced epilepsy only accounts for 1% of epileptic attacks, but it is the frequency of flashing rather than the light intensity which causes it. For this reason, a safety note about photo-induced epilepsy is given whenever any type of stroboscope is used.