Seeing Light Beams

When I see a beam of light going in front of me, what am I actually seeing? Are the photons of light leaving the beam and coming my way just to enter my eye so that I see the beam? Or is the beam visible because light beams from another source reflect off the beam in question and falls on our eye? If the latter is true, would this mean that a photon can reflect a photon? Further, will it be possible to see a laser beam (going in a perfectly straight line, and not deviating like a torch beam) in vacuum and in the absence of any other source of light?
- Savyasachi Jha (age 16)
New Delhi, India

You see the beam because the air has many dust particles in it and some of the light bounces off those particles heading various directions, including toward you.Although photons can bounce off each other, the process is extremely rare for visible photons in a vacuum or air. It plays no role in making the beam visible. Thus you won't be able to see the laser beam in a perfect vacuum.

 By the way, no beam can travel "in a perfectly straight line" since the wave equation (or, if you prefer, the uncertainty principle) always gives some spread in the directions the beam is going.

Mike W.

(published on 02/12/2013)