Why parts of the laser ray seem brighter than other parts

[titansarus]

Hi.

When we see a laser ray, it seems that part of the ray are brighter than the other parts and they seem like they’re vibrating and moving. It also seems there are fractures in its path. I want to know why do we see like this?

For example see this picture (it is from wikipedia). I circled some parts with pink. Why some of these parts are brighter than the others? And it seems there are fractures in its path (the rightmost circled one). Is it because of the procedure of making laser (stimulating) and that it is not a completely constant procedure in atomic scale?
Sorry for my English and thanks in advance.

 

[phinds]

probably differing amounts of dust in the air

 

[.Scott]

The only reason you are seeing the beams at all is that a colloidal dispersion (smoke, vapor (fog), or dust) is being used.
In the photo above, most beams are saturating the image - so variations in their intensity will not be rendered.
For the weaker beams, there are variations in the density of the fog.

 

[A.T.]

When we see a laser ray,...

... there must something in its way.

 

[Stavros Kiri]

The only reason you are seeing the beams at all is that a colloidal dispersion (smoke, vapor (fog), or dust) is being used.

You mean in vacuum you won't see the polarized beam at all. How can we test that here on earth?

P.S. (restatement of the first part): that means in vacuum you won't see the polarized beam at all.

 

[A.T.]

You mean in vacuum you won't see the polarized beam at all.

You would see it, if it hits your eye.

 

[Stavros Kiri]

You would see it, if it hits your eye.

Or see the dot when it hits an object. (My original statement (the first part) was not a question.)

 

[Stavros Kiri]

You mean in vacuum you won't see the polarized beam at all. How can we test that here on earth?

P.S. (restatement of the first part): that means in vacuum you won't see the polarized beam at all.

About the 2nd part: we can use a vacuum airtight glass tube and let the laser cross it. I just tried and it works. No beam inside.

[Or we can observe the beam changes for dimmer and dimmer air ... . That kind of explains (/is related to) also the original question:]

When we see a laser ray, it seems that part of the ray are brighter than the other parts and they seem like they’re vibrating and moving. It also seems there are fractures in its path. I want to know why do we see like this?

 

[phinds]

You mean in vacuum you won't see the polarized beam at all. How can we test that here on earth?

You've heard, maybe, of vacuum chambers?

 

[Stavros Kiri]

You've heard, maybe, of vacuum chambers?

You're late!

About the 2nd part: we can use a vacuum airtight glass tube and let the laser cross it. I just tried and it works. No beam inside.

 

[phinds]

You're late!

Yeah, not paying enough attention.

 

[Stavros Kiri]

Yeah, not paying enough attention.

Don't we all ... sometimes

 

[Shadow89]

probably differing amounts of dust in the air

This. Light always* travels in completely straight lines. For you to see the laser "beam" from the side, the light has to bounce off of something, like moisture or a speck of dust.
In film production, or at concerts, they will often use lasers in combination with dry-ice smoke machines and/or smoke from pyrotechnics to create the affect of a beam going through the air.

*(I don't know what happens to light when near black holes or other cosmic disturbances)