Freezed in motion
Purpose of the experiment:
Observation of moving objects illuminated with stroboscopic light. Finding a dependence between the appearance of the observed object and the flash frequency.
List of materials:
- a blower (a fan)
- a stroboscope with adjustable flash frequency (available at DIY stores, lighting shops or online, prices start at PLN 45)
- reflective film
- Glue a piece of reflective film (approx. 1 cm x 3 cm) to one of the fan blades
- Switch on the fan and the stroboscope.
- Observe the fan illuminated with stroboscopic light.
- Adjust flash frequency of the stroboscope.
Questions to the experiment:
- Commonly used fluorescent lamps and LED displays also emit light in the form of flashes. Why cannot they be seen with the naked eye? How can you show that a given source of light is also a ‘stroboscope’?
- Why can stroboscopic light be dangerous to man?
Description of the phenomenon:
Interpretations of results:
Objects around us are visible either because they emit light themselves or reflect (diffuse) light coming from an external source (e.g. a light bulb or the Sun). The rotating fan can only be seen at the time of a stroboscopic flash. Because the flash is very short, the movement of the fan blades is almost unnoticeable during the flash – we then only record their momentary position. Additionally, if we adjust the flash frequency to match the fan speed, then its blades will always be visible in the same position, which will give the impression that the fan does not rotate. A strip of reflective film will only enhance the impression.
- Stroboscopic light may trigger epileptic seizures in people.
- Stroboscopic effects are often used at discos and theatres in order to create the effect of intermittent movement of characters (stop motion).
- Photographs taken in stroboscopic light make it possible to study the individual phases of moving objects, e.g. car wheels, and thus facilitate detection of design faults.