Why is the path of a laser beam invisible after it originates, but once it is reflected by a mirror the path of the reflected beam is visible?
LASER is an acronym for light amplification by simulated emission of radiation. A laser beam is highly coherent and has a negligible divergence. This property of lasers makes a laser beam invisible in air, which is transparent. Only a point of light is visible where the laser beam strikes an opaque object.
Once a laser beam strikes an opaque target, a part of the energy is absorbed and the rest reflected. The reflected portion of the original laser beam does not have the same properties as the original beam that strikes the target. It is divergent and this makes it visible.