Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Stroboscope: introduction

Class practical

The hand-held stroboscope is a simple device that can be used in several very useful ways.

Apparatus and materials

Hand stroboscrope

Converging lens

Electric motor, fractional horsepower

Power supply, low voltage, continously variable

Retort stand and boss

Light source, compact

Lens holder

Rotating disc, motor-diven

Xenon stroboscope

Health & Safety and Technical notes

If a fractional-horsepower motor (Fracmo) is used in this activity, take care to connect up the field (stator) coils and the armature (rotor) coils before connecting both to the power supply. These connections should not be changed while the motor is running. 
The leads used to connect the motor should be fitted with 4 mm plugs having sprung shrouds (see the warning label enclosed with them). 

Photo-induced epilepsy 
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments. 
Teachers in maintained schools should check to see whether or not their Local Education Authority has given specific guidance on this matter.

The rotating disc is black, painted with a white arrow. 

The compact light source has an 8A low voltage power unit. 
Retort stand and boss are needed for both lamp and stroboscope. 
Xenon stroboscope is needed for one of the experiments.



To explain the principle of measuring a frequency with a stroboscope

Start by swinging your arm slowly round in a large vertical circle. 
Ask students to close their eyes and to open them briefly each time you say ‘now’, once each revolution. Students will see your arm each time in the same position. 
Then say ‘now’ once every two revolutions so they see the same thing but less often. 
Finally, say ‘now’ once every half revolution, so they see your arm in two positions. 
You can find the frequency of rotation from the maximum number of rotations of the stroboscope per second, which show your arm frozen in just one position; more positions and the stroboscope is turning too quickly. 
Summarize: the correct speed of rotation is the highest speed that ‘stops’ the object. The frequency of rotation is then the number of rotations of the stroboscope per second multiplied by the number of slits in the stroboscope. If the flash frequency is such that n stationary images are seen then the speed of rotation being measured will be N = (speed of flashes per minute)/n.(Thank you to Manoj Chouksey who suggested we include this sentence.) 
Students measure a frequency 
Use a motor to drive a black disc painted with a white arrow, at 25-30 revolutions per second. 

a motor to drive black discStudents should be able to rotate their stroboscopes at the correct speed. The number of slits passing the eye per second (12 glimpses per rotation multiplied by the average number of rotations per second) is equal to the number of revolutions per second of the disc. 

Another method – strobing with light 
Black out the room and use a lamp with a very bright small filament to illuminate the motor-driven disc.

Black out room and use lamp


Place a converging lens to form a real image of the filament of the lamp on the stroboscope disc. 
Now rotate the stroboscope disc, so that light flashes on the rotating disc at a rate that ‘stops’ the motion of the arrow. 
This shows an alternative method to rotating the stroboscope in front of the eye. 
Students measure mains frequency using a xenon stroboscope 
Set up a large neon lamp on AC mains. Do this experiment in daylight so that the lamp is visible even when the neon glow is not. 
Gradually increase the rate of flashing of the xenon stroboscope, until the lamp seems always to be ‘on’. This will be twice mains frequency, i.e. 100 per second, as the lamp lights with each voltage pulse. 

Teaching  notes

Using the stroboscopeIf a student cannot see the stopped motion, you can help her/him by working the stroboscope, looking through one side of it while s/he looks through the other.  

Hand stroboscopes are difficult to turn at high and low speeds. To show the effect of turning the stroboscope at half speed, and twice and three times the correct speed, you will need to run the motor at different speeds. Furthermore, at low speeds the white arrow becomes very spread out and indistinct, particularly at the edge of the disc where it is travelling fast. 
It’s easy to rotate the stroboscope at the wrong speed when measurements are being taken: 

  • Disc at 15 revolutions per second 

It’s difficult to turn the 12-slit stroboscope slowly enough to see a single stationary arrow. But if the stroboscope is speeded up until it is twice as fast, three times as fast, and even four times as fast, a stationary pattern is seen. 

  • Disc at 50 revolutions per second 


It’s possible to ‘stop’ the motion of the arrow by turning the stroboscope at the correct speed, at half this speed, and at one-third of this speed. It’s not, however, possible to check that the highest of these three speeds is the correct speed – it is too difficult to rotate the stroboscope fast enough to get the ‘twice-as-fast’ pattern. 

Some examples for discussion or for student investigation: 
1 Wagon wheels on the cinema screen. 
2 A cinema screen itself through a stroboscope. 
3 Fluorescent lighting or street lighting through a stroboscope (calculating its frequency would require a 24-slit hand stroboscope.) 
4 Hi-fi turntables (for old-fashioned vinyl recordings) turn relatively slowly. Some models have a large number of radial white bars marked near the circumference. When the speed is correct, each bar moves ahead one place for each flash of the mains lighting (100 flashes/second). 
5 A fan with several blades can be ‘stopped’ with various speeds of the stroboscope. If one blade has a white marker, however, it becomes apparent that many of these speeds do not give the actual speed of the fan. 
6 Observe the back wheel of an inverted bicycle through the stroboscope. Many speeds of the stroboscope appear to stop the wheel if the spokes look alike. 
This experiment was safety-checked in August 2006