1MHz electrical pulses generated by a built-in oscillator are transformed into light pulses and simultaneously applied to two optical fibers, one long and one short. Their length difference is 20m. After traveling along the fibers, the light pulses are transformed back into electrical pulses and applied to the two channels of an oscilloscope, where the time delay between the arriving pulses caused by their different path lengths can be seen, measured, and used to calculate the velocity of light. The apparatus includes the 15 x 11cm electronic board, a 9V DC power supply, 15cm & 20m fiber optic cables with efficient "sweet spot", and a full instruction manual.You need to supply a 30MHz dual channel oscilloscope and banana plug adapters for connecting to the 2mm circuit board jacks.