Measuring the speed of light
The experimental device - RLC platform The experimental device - RLC platform

The experimental setup

Describe the experimental setup used. For this purpose you can use Figure 3 schematically showing the experimental setup for measuring the speed of light to be used, and three photos of the device.

Light pulses lasting 20 ns each, frequency 40 kHz and wavelength 615 nm emitted from a high power source LED (1) located in an electro-optical unit (2). The light passes through a semipermeable mirror (3) and a portion of propagated towards which is disposed outside the electro-optical unit, a stationary mirror (4). The remaining light passes through a lens (5) and propagates in the direction which is placed a mirror (6) which can be moved, which is also outside of the electro-optical unit. Both pulses are reflected back to back and detected by a photodiode (7) which converts them into electrical signals.

Figure 3: The experimental setup and the various parts. High power LED (1), electro-optical unit (2), semipermeable mirror (3), fixed reflector (4), lens (5), removable mirror (6), LED (7), oscilloscope (8), children's train (9), wheel (10) with light barrier (11), removable baffles (12), camcorder (13), camcorder (14).


Because the difference between the road traveled by the two pulses is 2s, the light pulse reflected from the movable mirror reaches the photodiode later than the other pulse. The delay time is displayed on an oscilloscope screen (8), which is connected to the electro-optical unit. The same display also the signal from a square pulse frequency 10 MHz. This pulse allows us to calibrate the horizontal time axis and thus to measure the time interval between the arrival of the two pulses in the detector. The movable mirror is mounted on an electrically driven and controlled train kids (9). To measure the distance s the movement of the train carried by a wire to a wheel (10) with with light barrier (11).

The time measured is sufficiently accurate only if the two signals have the same height on the oscilloscope screen. Two movable diaphragms (12) that interfere with the course of the two light beams used to adjust the characteristics of both signs. With a camera (13) one can monitor the screen of the oscilloscope and take pictures, while a second camera (14) can monitor the movement of children's train.

The RLC platform

To make these measurements will use the platform RLC.

After opening with the browser of your choice page: 

select the language in which you want to "run" your measurements by selecting the corresponding flag. Suppose you choose English (Figure 4).


is a list of experiments that you can access. Select the Speed ​​of Light experiment and then to gain access to the lab select Laboratory.

Figure 4: The first page of the workshop RLC.



It is not compulsory to provide your details to gain access to handle the interface.

The interface control of the laboratory shown in Figure 5.

Picture 5. The interface control of the laboratory


UniSchooLabS is funded with support from the European Commission.
This document reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein.