Example Setup, Speed of Light

A laboratory speed of light experiment is conducted by directing ight from a modulated He-Ne laser is detected at the beginning and end of a measured path. The AC signals from photodetectors at these two points are compared on an oscilloscope to measure the phase difference and from that the velocity of the light.

A beam splitter is used to divide the beam for detection at begining and end of the measured path. Plane, front-surfaced mirrors are used to deflect the beam to get as long a path as possible. 分束器用于将光束分成两部分，以便在测量路径的开始和结束处进行检测。平面、前表面的镜子用于将光束反射，以尽可能延长路径长度。

Experimenters: Chris Cleven, Chris Farrington, Ryan Webb 实验者：Chris Cleven, Chris Farrington, Ryan Webb

After reflection from the distant mirrors, the modulated beam is detected and compared on an oscilloscope to the beam which is reflected from the beam splitter. The speed of light can be computed from the measured time difference. 在经过远处镜子反射后，调制的光束被检测并用示波器与从分束器反射的光束进行比较。光速可以从测得的时间差计算出来。

The time interval measured on the oscilloscope display above is 70 nanoseconds. For the longer path illustrated in the sketch above, the measured time interval was 110 ns and corresponded to a light speed of 2.76 x 10^8 m/s.