An LED (pictured above, far right) emits an invisible pulse of light which is passes through two lenses and into a photo multipler tube (pictured far left). From the tube the signal is passed into the oscilloscope. The signal that causes the LED to pulse also triggers the oscilloscope. Moving the LED along the optics rail causes the signal displayed on the oscilloscope to also move. Thus, the distance the LED is moved can be correlated with the time scale of the oscilloscope. A one meter long plexiglass rod can be placed on the rail in the path of the light to allow comparison of the speed of light in air and in plastic.