Light traveling from point A to point B cannot be seen directly unless photons are scattered toward the observer's eye.
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Physics discussions and expert explanations confirm that a beam of light traveling through space cannot be directly seen unless photons scatter or reflect off particles toward the observer's eye.
# Why can't we see light travelling from point A to B?
Tags: visible-light, photons, astronomy, speed-of-light
- Score: 37
- Views: 16100
- Answers: 8
- Answered: yes
- Asked by: Zer01 (405 rep)
- Asked: 2015-07-29
- Edited: 2015-08-01
- Site: physics
## Question
Let's say we have a cloud of dust which is a lightyear across and someone shoots a beam of light from point A to B , why it is not possible for an observer far far away to see the light while it travels through the cloud at the speed of light?
## Answers
### Answer by user12029 (score: 75 [ACCEPTED])
Sometimes we do, and the phenomenon is called a light echo.
What you're looking at there is NOT moving gas. It's an "echo" exactly as you describe.
The problem is that you need a pulse of light. If you have a constant stream of light, the "light echos" will be exactly like what you see in fog on earth.
### Answer by Ernie (score: 24)
It would be possible to see the progress of photons through space if the light pulse were exceedingly intense, and if the dust cloud from which they reflect were positioned and shaped to reflect the light toward us. Rather than shooting a beam from Point A to Point B, it would be better
# How can we see a beam of light?
Tags: photons, everyday-life, vision
- Score: 17
- Views: 7355
- Answers: 1
- Answered: yes
- Asked by: Cazo (373 rep)
- Asked: 2018-08-02
- Edited: 2018-08-02
- Site: physics
## Question
A beam of light is made of photons, which simply travel in a line from point $\text{A}$ to point $\text{B}$. But we can only see things when photons hit our retina, so doesn't this mean that the photons of the beam ought to travel to our eyes? How is it possible to see the beam?
## Answers
### Answer by Time4Tea (score: 56 [ACCEPTED])
You're not seeing the photons in the beam that are traveling from A to B (beam starting point to beam destination), you are seeing photons that are scattering off of dust particles that are in the path of the beam.
This is the reason why you see lasers in a night club more clearly when there is a smoke machine, and why cat burglars blow dust onto security beams, to expose them ;-)
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