# Why are green laser beams visible?



## Fallingwater (Dec 19, 2007)

I only have cheap red pointers so far, and only the red dot is visible unless you spray something in the beam's path.

I know that with green pointers you can see the beam. Why? What's the difference inbetween them that causes red beams not to be visible?


----------



## Kiessling (Dec 19, 2007)

Power. The green beam, just like the read beam when you spray things in its path, lights up those littel particles. As the human eye is way more sensitive to the green light, it appears much brighter than the red at a given power level, and you can see the illuminated particles better.
A high-powered red beam is visible, too.
bk


----------



## nanoWatt (Dec 19, 2007)

Don't forget in a vacuum, that no matter how powerful or color, you won't see the beam of a laser unless something scatters it. So those Star Wars battles in space are obviously fake. Without any dust, you wouldn't see the beam until it hit you, or the dot bounced off a surface.


----------



## Tachikoma (Dec 19, 2007)

It's all about the wavelenght: human eye detects various wavelenghts starting from 400nm (near ultra violet) down to about 690nm (near infra red), obviously the more you get closer to these limits the less you can see the light, so around the middle at about 555nm (light green) you can see the light itself better.
Normal red diode pointers emit at 650nm while green pumped do it at 532nm, so to see the dot at 10km on a white surface at night you'll need a green 1mw or a red 5mw, to see the beam itself without using fog or smoke at night you'll need at least a green 5mw or a red 25mw.
I'm not saying the exact red/green visibility ratio is 1:5, but it should be fairly close to reality according to my own experience on the field.
Dear Falling, let me know when you'll come near Powercity and I'll show you my babies:thumbsup:


----------



## Fallingwater (Dec 20, 2007)

Uh... what's Powercity? :thinking:


----------



## TorchBoy (Dec 20, 2007)

Kiessling said:


> Power.


Nup. Well, yep, but nup.



nanoWatt said:


> Don't forget in a vacuum, that no matter how powerful or color, you won't see the beam of a laser unless something scatters it. So those Star Wars battles in space are obviously fake.


Nup. Those Star Wars battles aren't with lasers. And it's light sabre, not "laser" sword.



Tachikoma said:


> It's all about the wavelenght:/QUOTE]
> Yep. Depending on what the relative wavelengths are 1:5 is about right. So green light is easier to see than red light with the same power.
> 
> There's also the scatterability of the light. Green light scatters more easily than red light, and blue light scatters even more easily still (and is why the sky is blue*). So you can see the beam of blue lasers even more easily, although the eye is less sensitive to that colour.
> ...


----------



## nanoWatt (Dec 20, 2007)

Thanks for the clarifying on the light sabre. I was referring to the ship battles, where they fire pulses of what seems like lasers. But they could be particle weapons. Anyway, if you could see them, let's say like on a planet, the pulses would be too fast to really notice, since light moves around 186,281 miles per second last I checked). The pulses were even slower than sound. Obviously just for Hollywood effect.


----------



## Tachikoma (Dec 20, 2007)

Fallingwater said:


> Uh... what's Powercity? :thinking:


Potenza, south of Italy XD
The babies I was referring to are:
a red 5mw keychain (paid L.100'000 in 1996  )
a green 2mw (paid 75Euro for a fake 20mw 2 years ago  )
a Wickedlasers Core (true 5mw green)
a Wickedlasers Nexus (95mw green)
a Wickedlasers Spyder II GX (~200mw green)


----------



## comozo (Dec 20, 2007)

Rayleigh scattering and human vision is most sensitive too the green portion of the visible spectrum


----------



## nanoWatt (Dec 20, 2007)

I thought human vision was most sensitive to yellow. And even more sensitive to white (not a part, but the whole spectrum).



comozo said:


> Rayleigh scattering and human vision is most sensitive too the green portion of the visible spectrum


----------



## Tachikoma (Dec 20, 2007)

As you've said white light is composed of all visible colors, but if you take a white light and a light that emits at 555nm (light green somehow tending to yellow) you'll notice that your eye will see the 555nm one even brighter and more visible than the white one.


----------



## The_LED_Museum (Dec 20, 2007)

nanoWatt said:


> ...since light moves around 186,281 miles per second last I checked).


Last time I checked, light travels at 185,999.998 miles per second - though I do not know if this is in a vacuum or at 1 atmosphere of pressure.


----------



## nanoWatt (Dec 20, 2007)

When I read it like 10 years ago they were saying 186281.7 miles/sec.



The_LED_Museum said:


> Last time I checked, light travels at 185,999.998 miles per second - though I do not know if this is in a vacuum or at 1 atmosphere of pressure.


----------



## comozo (Dec 21, 2007)

nanoWatt said:


> I thought human vision was most sensitive to yellow. And even more sensitive to white (not a part, but the whole spectrum).



Peak photopic sensitivity is 555.5nm


----------



## comozo (Dec 21, 2007)

nanoWatt said:


> I thought human vision was most sensitive to yellow. And even more sensitive to white (not a part, but the whole spectrum).



post x 2


----------



## TorchBoy (Dec 21, 2007)

nanoWatt said:


> I was referring to the ship battles, where they fire pulses of what seems like lasers. But they could be particle weapons.


It takes more than a glow to make a laser. As you pointed out, they can be seen from behind as they travel. Also they travel much slower than light. Particle weapons sounds more reasonable.


----------



## monkeyboy (Dec 21, 2007)

I also find that green laser scatters more easily than red. I sometimes work in a lab with a 700mW Krypton ion laser (actual measured power). You can see the larger particles of dust in the beam but it doesn't seem to "trace" the beam like it does with a green laser. I think it's not just that eye is more sensitive to green light but also that the green light is scattered more by the smaller particles in the air.

We also have a 10W green laser that looks like something out of Mission Impossible. Unfortunately I don't get to play with this.


----------



## nanoWatt (Dec 21, 2007)

Green light also has a shorter wavelength than red. So this means more waves of the green can hit the speck of dust to be scattered.


----------



## Fallingwater (Dec 21, 2007)

Please stop trying to apply science to movies.


----------



## TorchBoy (Dec 21, 2007)

Fallingwater said:


> Please stop trying to apply science to movies.


Well, we got that sussed, got bored, and moved on. :wave: But Star Wars does rather defy rational explanation.

According to Wikipedia, light travels at "about 186,282.397 miles per second, or roughly one foot per nanosecond." That's in a vacuum.


----------

