Why do lasers look stronger coming at you?

[laserLips]

OK this may be a bit laser 101 but I've never heard or found the answer.
So please, So why do we see beams coming at us better than beams going away?

My logic says that because we see light that's reflected of stuff, we would see beams traveling away from us better because we see 'this' side of whatever the beams is hitting (dust, water etc).
But nooOooo. So why then?


Graham

[mixedgas]

Raleigh and Mie Scattering have complex rules. So which set applies depends on the particle size relative to wavelength.

Find DOT-TSC-FAA-79-22 for a government published paper about visible laser scattering in dense fog.

I've read it before but I am unable to technically explain or comment as I am traveling.

PS, As for what is in the paper... Don't try that at home kiddies, the reason scattering based LS are not used is the patents are so sown up, you'll be sued before you even order the parts. Many, Many, confusing patents and a history of instant law suits.

Steve

[DTR]

My guess would be as a photon moving in a straight line and hits a particle in the air the chances of it hitting it dead on and being reflected backwards is much lower than an off center collision that just slightly alters its trajectory.

[Steve Milani]

I am not a physic....
This is my personal thought only.


Because the phenomenon does not occur in conditions of absolute vacuum.

Is the "Principle of energy conservation". Physic law.

The beam has a certain amount of energy when it comes out from the source that has generated it.
Step by step, the beam moves and loses an infinitesimal part of its energy (electromagnetic radiation), caused by collisions with other particles in the air. (in the air = all that is around it)

That's why the radius, as further away goes as more it loose brightness (and widens, increasing divergence). Because it loose energy.

In the "absolute vacuum", the "Principle of energy conservation" wins: no any sort of obstacle, no loss of energy.
Theoretically (and practically also) in the cosmic deep space is 100% absolute vacuum. There is present nothing of nothing.
This means no any other kind of energy is present. Nothing can impede at the laser energy to go on. Nothing can make him lose its energy.

That's why we can see, by our eyes, the light coming from stars "light years" far from Earth. Because in the deep space nothing, or almost nothing (star dust...) can impede to the "energy of the light" to go on toward our eyes.

I hope I have not written a.. bullshit.
If the case I apologize

Greets!

[laserist]

OK this may be a bit laser 101 but I've never heard or found the answer.
So please, So why do we see beams coming at us better than beams going away?

My logic says that because we see light that's reflected of stuff, we would see beams traveling away from us better because we see 'this' side of whatever the beams is hitting (dust, water etc).
But nooOooo. So why then?


Graham

I believe diffraction is a significant part of this effect. a photon doesn't have to strike a particle in the air it just has to come close enought to diffract it's path a bit...

[The_Doctor]

I wrote a post mentioning diffraction when I first saw this thread, and after posting it saw Steve's post (he posted it while I was writing mine) and I deleted mine because it looked so vague and probably wrong, but I'll have another crack at it. It seemed to me like the diffraction of waves in a gap in a harbour wall. Most of the energy keeps close to the original direction, diverging only if it must. As to the second strongest view, of the departing beam, I'm guessing that is reflection.

Edit:
I think the difference in strength of scatter from equal beams coming or going probably needs to be compared at same angle. I suspect that in the departing beam the brighter appearance seen when closer to the beam is mostly because the same volume of air is seen in a much smaller solid angle so more light appears to come from some specific small solid arc of that view. So measuring the difference between forward scattering and anything else presumably has to take that into account.

[andythemechanic]

If you have very small scattering particles (much smaller than wavelength of light --> Rayleigh Scattering), then the scattering is independent of the angle and a scattered beam would look equal bright from all directions. If the particles are very big (>some µm, the usual case in laser projection) then the forward scattering effect is easy to understand: a big almost spherical partical just behaves like a lens, focussing the impinging beam towards it's propagation direction.

[Galvonaut]

Is this why fine oil haze is more effective than heavier fog?

If you have very small scattering particles (much smaller than wavelength of light --> Rayleigh Scattering), then the scattering is independent of the angle and a scattered beam would look equal bright from all directions. If the particles are very big (>some µm, the usual case in laser projection) then the forward scattering effect is easy to understand: a big almost spherical partical just behaves like a lens, focussing the impinging beam towards it's propagation direction.

[The_Doctor]

Is this why fine oil haze is more effective than heavier fog?

It might just be because haze is more uniform and stays around longer. I read that the particles were about 1 µm so about the same as 1064 nm pump diode light. All visible wavelengths will be smaller, not bigger as they'd need to be.

Edit:
Andy, that lens bit is interesting, I didn't think of that. But I notice the forward-brighter thing in dry air with various sizes and types. I used to leave a diode laser shining across a room on one side for months at a time because I liked looking at the beam when I was resting, and it got especially entertaining after frying sausages, or if someone a few houses away lit a bonfire. I could gauge pollem levels and air pollution from it too. I bet that diffraction happens a lot more than refraction...