Light speed measurement apparatus

A desktop apparatus for the accurate determination  of the speed of light in free propagation.

  • Quick installation: 45 seconds for connecting the cables to the control unit and the power supply to the mains socket.

  • Fully autonomous: it does not require any instrumentation or auxiliary optical components such as oscilloscopes, computers, data loggers, function or pulse generators, power supplies, lenses, supports, etc.

  • Nothing to align: apparatus immediately ready to perform measurements.

  • No small detachable and easily loose part.

  • Determination of the speed of light c in free propagation by measuring the space traveled by light by means of the modulation of a laser beam at a frequency of 433.9 MHz and of the detection of the phase of the optical signal received at a certain distance. It is the shortest of all existing commercial equipment: 70 cm.

  • Direct determination of the speed of light c in transparent or almost transparent solids in the form of bars a few centimeters long, with the ends approximately flat-parallel and glossy, as well as in liquids and their solutions contained in test cells with a volume of about 10 cm3. The cell is supplied with the equipment.

  • Determination of the relative refractive index n of solids or transparent or almost transparent liquids following the definition: n = c / v.

  • Determination of the propagation velocity of the signals in cables and of the dielectric constant of their dielectric. A test cable is provided.

  • Display suitable for a large audience.

  • High accuracy on single measurement: a relative uncertainty less than 0.01, which can be reduced with the use of standard measurement techniques.

  • The working principle is simple and intuitive.

  • Transportable version, for demonstrations in the classroom, for exhibitions and for measurements by the students.

  • Small-sized packaging for convenient transportation. The optical bench is 140 cm long and housed in a cylindrical container of 8 cm in diameter, the other component parts in a sturdy cardboard container easily handled.

  • Total weight including the optical bench about 2 Kg.

A desktop apparatus for the accurate determination  of the speed of light in free propagation (accuracy is 0.1% for a single measurement).

This apparatus is
– highly portable (fits into a small suitcase and in a tube of 130 cm length),
– pre aligned (no alignment necessary)
– quickly operative (setup in less than 1 minute)

It allows also measurements of:

– refraction index of solids and of small volume of liquids
– “velocity factor” of coaxial cables

No oscilloscope, no generator, no auxiliary equipment is required.

The apparatus’ sensitivity is so high that it detects the slowing of light
as it passes through the thickness of a microscope slide (about 1 mm )

See: G. Pegna G. Pegna , An extraordinary tabletop speed of light apparatus American Journal of Physics 85, 712 (2017)


Large version for museums and science centres on request.