Why do green lasers rays are visible?

fluidistic

I've seen quite a few videos in youtube (like http://www.youtube.com/watch?v=woiTe...feature=relmfu ) and I wonder why we can easily see the laser in the air, as if there was some diffusion of it in the air.

Gordianus

You're just seeing light scattering. The same happens when you see a beautiful blue sky. Search Rayleigh and Mie scattering.

fluidistic

But then why red lasers are totally different? I mean I don't see the ray but only the points where the ray hits an object.

pergradus

One, redlight is scattered less efficiently than shorter wavelengths like blue and green.

Two, your eye is more sensitive to green light than to red.

fluidistic

I consider the first reason to be the strongest one (as I don't see absolutely any ray with red light, not even in the dark). Impressive, really impressive.
Thanks guys.

Gordianus

Get a red laser pointer, turn off the lights and watch the beam sideways. You'll see scattered light again. The difference stems from the fact that scattering cross section is large for green light and your eyes are more sensitive.

BruceW

It is possible to see the laser in the air from red lasers also. (I've seen it).
I doubt it is the first reason. Its true that the red light from the sun gets scattered less (causing red-coloured sun in the evening and morning), but this is over a huge amount of air. I would guess that the effect of different wavelength over short distances to be negligible.
The second reason is possible - never underestimate the affect of human perception on what we see! :)
But maybe it could be due to the power of the laser? Or the difference in the way the particular laser works? (i.e. one laser may produce a slight spread in frequencies, which would be easier for us humans to observe from scattering)

fluidistic

I guess there was smoke or anything that could cause some dispersion.
Very interesting point! I hope someone else -expert- can confirm this.
If we can see the ray without having it hitting our retina means the ray has been scattered or dispersed in the air. Otherwise we wouldn't see it.
Another interesting point, though some green lasers in youtube are 0.5 mW and are still visible. This is a low power IMO. So I'm not so sure it plays a big role, but I doubt.
Usually lasers have a very, very tiny spread in frequency so the answer here is a clear and big no.

fluidistic

I don't have a red laser pointer unfortunately. I'd have to search in youtube some video about red laser in the dark. As of now, I've found http://www.youtube.com/watch?v=z6oG2vW-blM but the ray is totally invisible, unless some smoke passes in the ray.

Gordianus

Then check entries like "He Ne laser water waveguide". I've seen nice videos showing scattered red light in water

BruceW

I just calculated the difference in scattered light (after travelling 1 meter) from a red and blue laser due to Rayleigh scattering (thanks to wikipedia for the number densities, equations, etc). And it turns out that the scattering is roughly 10 times greater for blue light. So this does seem likely to be the reason why green laser light gets scattered more than red.
But you can also see the laser in the air from red lasers - I saw it with my own laser, not just on a video on youtube, and there was no smoke around at the time :)

fluidistic

Ok I take this as a good answer :)
I was hoping for a much greater factor since... hell I couldn't see AT ALL the red ray of a laser in a dark room. But maybe humidity plays a big factor too.
So I think my question is answered. Thanks guys.

cjl

I would guess that it's primarily the low sensitivity of the eye to red light. I have a 0.5mW violet (405nm) laser, and its beam isn't visible at all either. Green lasers are in a region where your eye is extremely sensitive, so even at fairly low power, they appear quite bright.

Geezer

As a side note, human eyes are naturally more sensitive to the yellow and green portions of the visible EM spectrum than to red. It's not enough to account for the entire difference between the perceived intensities between the red and green light, but it makes a contribution.

BruceW

This is a picture from wikipedia, showing the sensitivity of the eye as a function of wavelength. The green curve represents the sensitivity at low light levels (which is what I assume is true in our case).
It seems to suggest that the sensitivity to red light is 100 times less than green (at low light levels). That's a massive factor!! I guess that could be right, since I could only just see my red laser, but you say that the green laser was easy to see.
The black curve shows the sensitivity of the eye at normal light levels. I guess this is what to expect if it was a really powerful laser, then the eye could see red laser light more easily.

Attached Thumbnails

 

Dever

I believe this is an important contribution. Most likely that website will also detail the difference between he eye's absorption mechanism of red and green light. Seeing red in the dark vs. light should not be substantially different since the eye's saturation to red light doesn't scale exactly like it does with other light, with the exception of the effect of pupil dilation. Red wavelengths are only absorbed by the cones in the eye, which are 100 times less dense than rods if I remember correctly, which explains the sensitivity difference, and the reason why our eyes don't have to adjust to red wavelengths in the dark.

Additionally, Rayleigh scattering is important, but not the only factor that changes with wavelength. Not just the scattering, but the absorption of red, IR, etc., should be more probable because they require a lower energy transition than higher energy light like green, blue, UV, etc.

If you're considering solid-state lasers, you might also want to check the effects of the resonator bandwidth and coherence on the scattering, because SS red lasers are much easier to come by, so SS green laser producers will not seek a smaller bandwidth at the expense of power.