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# Why incident beams brighter?

#### Meatball

##### New member

Why is it that a beam is brighter when the beam propagates towards you, than when you stand behind the source?

I ask because, I though that brightness would be a function of the number of particles in the air that reflect the incident wavelength, the size of said particles, and the angle of incidence of the beam to the particles.

So I thought reflection off these tiny particles would most often result in a change in direction that is more LIKELY to be 180 degrees.

What am I missing here? Is this an effect determined by "scattering"?

#### Trevor

##### Well-known member

If you fire a bullet at a steel sphere, it's far more likely to glance off and keep moving forward than it is to bounce back at 180 degrees.

Not sure if light works the same way, but it seems to make sense to me. Then again, the geocentric theory made sense at the time too...

-Trevor

#### JBTexas

##### New member
For that specific question; I only have a guess. I think it's a good guess though.

Yes, scattering.

If you are aiming, with a low angle of incidence, AWAY from you; you see the beam because dust particles, smoke, etc., are lit by the beam.

If it is TOWARDS you; dust, smoke, etc is semi-transparant, and you still get the same effect, more-or-less, but you are also seeing pronounced SCATTERING.

Any given dust particle, when lit from "behind", will just be lit up.

When back-lit, light shines around the edges and illuminates the edges, but also shines THRU, with dispersal, so all the particulates are more uniformly (and more densly) lit.

#### JBTexas

##### New member
Ooops... sorry Trev... just noticed... we posted at the same time. See, posts can collide and scatter too!

(re: Angle of Incidence); It's probably a little of both, and then some.

#### Cyparagon

##### Well-known member
There will also be a small amount of diffraction around each particle.

#### Legion93

##### New member
Divergence. The beam gets scattered at a particular distance therefore it appears to have a wider diameter in the beam as the distance increases. Let's put it this way, If you look at the sun (don't! It's dangerous and hardly possible!) it is going to irritate your eyes because the light rays pass and hit directly to your eye.

It is just like when someone points a flashlight at you, you are totally visible to them, however you don't see them.

On another note, when you look at the laser from the front it appears brighter due to its light source from the diode and the beam combined. If you stand behind the laser, you cannot see the light coming out from the diode, however you can see the beam so it makes it seeing one light source compared from 2 if you stand in front.

It's kind of like a visual illusion.

I doubt it has anything to do with angles and such, just the way you look at the laser.

I agree with post above ^^ about the effect of particles and dust/smoke.

Did I express my writing clear?

#### Benm

##### Well-known member
It depends on the size of the particles. If they are very large, you will in fact see little or no beam when it comes at you - imagine something like bouncing tennisballs in the path.

With particles that are small compared to the wavelength, the opposite happens: they "deflect" the photos only slighly off course, resulting in a beam more visible if it comes towards you.

Its important to know that both effect happen at the same time under normal circumstances, which causes the beam to be most visible when coming at you, but also when going away from you, compared to looking at it from the side.

In perfectly clean dry air, i would assume that the beam going away from you is actually less bright then when viewing it from the side, though i do not have a cleanroom at hand to test this

#### qumefox

##### New member
Legion. Divergence has nothing to do with scatter.