Laser is used for this method. The word laser stands for light amplification by stimulated emission of radiation. Laser is a source of very intense, highly monochromatic (of one wavelength) and highly directional beam of light.
A laser beam is sent towards the moon and its reflected pulse is received. If ‘t’ is the time elapsed between the instants the laser beam is sent and received back, then the distance of the moon from the earth is given by
S = c x t / 2
where c = 3 × 108 ms-1, is the speed of light