How bright is a laser beam when viewed from the side?
Category: Space Published: February 14, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
While traveling through the vacuum of space, laser beams are invisible unless shot directly into your eye. The experience you know of as vision consists of light entering your eyes and being detecting by cells on your retinas. You can't see any light that never enters your eyes. A beam of light, including laser light, will not enter your eye unless aimed directly at it or reflected directly into it by an object. The vacuum of space does not have anything to reflect the light back into your eye. Only by adding air, dust, or debris does a light beam become visible from the side. This should be obvious to anyone that has every used a flashlight. The objects that reflect the light from your flashlight are visible, but the beam itself traversing through clean empty air is not. Laser beams are no different from flashlights in this regard. Anyone who has every used a laser pointer to give a presentation can attest to this fact. When a movie hero shoots laser beams from his spaceship, none of these beams should be visible as they travel. I believe entertainment producers get this principle wrong on purpose for dramatic effect. It's much more dramatic to see a flash of light shoot out of a gun than nothing visible shoot out.