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Reason for the beam being way more visible from the front?

Yelov

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Jan 20, 2019
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I have a 100mW 532nm laser, and was just wondering why the beam visibility is way different from looking at the beam from the point of view of the laser vs looking at it from the other side (to the laser). It's just scattering of particles in the air, right? Thought it would go in all directions equally. For example I can see the beam in daylight when looking from the end, but absolutely no beam when looking from the front. I've never seen this in the YouTube videos I've watched.

edit: nevermind, just found a video where it's visible

laser1.jpglaser2.jpg
 
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chloderic

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There is realy a great difference in scattering , reflection and diffaction.
Reflection backwards and scattering backwards is not so strong as the diffraction in the beam direction ...
 

hakzaw1

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Its one of 'those things' we do not question. "tis wat it tis''
best.. coming at you (& most danger)

good.. looking 'down' the beams- coming from behind or looking from behind .

worst .. from the side..

some may feel different about 1 and 2 but we all agree about laser viewing from the side. ng
 

smallfreak

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Thought it would go in all directions equally.
If you imagine the particles as big, shiny spheres, you will expect a fairly even distribution of stray light in any direction. However, the world usually does not provide us with perfectly shiny and spherical dust grains and light travels more complicated than straight lines, when interacting with matter.

Gracing a dust grain, the wave nature of light makes it bend a little around the grain. The new path is a little off to the side of the grain, but still forward. It does not swing around shining backwards. This diffraction light can be seen from the front (looking towadrs the Laser) but not from the back (looking into the pointing direction). In addition, even quite dark (or transparent) particles are reflective when hit at a very shallow angle, deflecting more light off the path in forward direction, just a small angle. Reflecting backwards needs a center hit on the particle and a shiny surface. You still get a chance that the ray simply gets absorbed by the material. To get the light perpendicular of to the side, the surface of the grain must be hit at a precise angle, where the chances are comparably bad.

Putting this all together you will get the most stray light a few degrees off the nominal direction, followed by backward reflections. The chance to see a particle frm the side diminishes rapidly with growing angle.

All that said, the exact scatter profile depends heavily on the actual shape pf the particles, so this can be used to determine the shape from measuring the scatter profile. An interesting task when at times the air is full of pollen or ice crystals that produce all kinds of different halo.
 

RedCowboy

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This is why air that's dense with water vapor ( Fog ) makes a beam look so much brighter and in the empty vacuum of space you wouldn't see your beam at all or extremely little depending on how much micro particulate is scattered around say....a planet or in a planets orbit, but in empty space there's nothing to show the beam.

This is why when taking a beam pic I like to burn a piece of wood or paper/cardboard......just for a few seconds and then stir that little bit of smoke into the air, it really shows the beam without clouding up the pic.
 




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