does air reflect light?

[coke]

Quick question, really no reason to go any further than this...

 

Does air reflect light?

i.e. if you shine a very strong beam of light through normal air (with absolutely no dust) will you see a faint beam of light?

Or if you use a lens to focus that beam of light onto a single point, will you see a bright point?

Oh and if it absorbs and re-emits the light, thats just as good...

 

I need it for my hologram design!

[insane_alien]

no. it doesn't although if you have a beam with an anergy density of over a megajoule per cubic centimeter you will get blooming which is visible. not what you want though.

[swansont]

Reflection happens at interfaces between media if differing index of refraction. Typically when we say something reflects, we say it's caused by the material with the higher index. So technically speaking it would if you could have an interface with a lower index. You're pretty much restricted to a dilute gas or vacuum (or perhaps helium or hydrogen)

 

[math]R=(\frac{n_1-n_2}{n_1+n_2})^2[/math]

 

But a distinct boundary between air and those gases, or a vacuum, would be exceedingly hard to achieve

 

Pedantically, though, there is a solution. If you liquified the air you would have a reflection.

 

The situation described, though, is not reflection since there is no interface. What you describe is scattering. Light does scatter in air which is why the sky is blue (Rayleigh scattering) but this is very weak on the scale under discussion.

[coke]

What about something like idk, xenon, so if you focus a lot of ultraviolet lasers or something it will re emit in normal visible light?

[Klaynos]

I've a green laser used for pumping my main laser, and that one you can clearly see the green beam path floating in the air due to the scatter from the air... It's in a clean room so there is very little dust.

[coke]

great, thanks! So with little dust it will shine powerfully enough...

 

now I wonder, if I were to focus a powerful laser onto one point, could I get a brightly shining point? Without the laser's beam being too visible?

 

What's IRC?

Edited April 5, 2009 by coke

[Klaynos]

The one we use for experiments you can focus it quite easily so you can see the focus floating in space... Need to get a photo of that at some point...

[Cap'n Refsmmat]

[stereologist]

There are dyes called 2 photon dyes that require 2 photons to hit at once to fluoresce. Two intersecting laser beams in a vat of dye will illuminate a single point in the tank.

[Kyrisch]

Does air reflect light?

 

Well, air reflects light to some extent, otherwise the sky wouldn't be blue. Granted, it's barely significant, but still.

[coke]

nice one cap'n, how many mw is that?

 

that's an interesting idea, stereologist, in fact i think they're making a hologram dvd disc on a similar concept... maybe not, idk

 

but that would work wouldn't it? say a 10-layered hologram disc with these dyes replacing the simply reflective coating: first two lasers focuse on the bottom layer, and after each read, focus on the layer above it... so 10 times storage of a dvd... or 10 times storage of a blu-ray for that matter...

[Cap'n Refsmmat]

The description of that image claims it's a 5mW laser. I'm guessing it's a relatively long exposure time to show up that well. My 50mW laser will do that for the naked eye at night.

[coke]

The description of that image claims it's a 5mW laser. I'm guessing it's a relatively long exposure time to show up that well. My 50mW laser will do that for the naked eye at night.

 

Ah, that's interesting... I went to an electronics store and there were a bunch of red laser pointers (you know the ones you play with as a kid) and an expensive supposedly very powerful green laser... (of course, prob. not real lasers, just a led + lens)

 

But all said max. output is 5mW, so it surprises me how much the frequency difference or whatever has an effect...

 

Wait a sec!

 

Just wikipedia'd laser pointer and guess what picture came up?

Edited April 6, 2009 by coke

[Cap'n Refsmmat]

I think the difference is partly because the human eye is more sensitive to green wavelengths than red.

[Klaynos]

The description of that image claims it's a 5mW laser. I'm guessing it's a relatively long exposure time to show up that well. My 50mW laser will do that for the naked eye at night.

 

My pump laser which does this has a beam diameter of about 5cm, and is 45W (and yes that's 45W, it's not a typo).

 

It is actually easier to build a semiconductor laser than it is to build a well collimated source from an LED and a lens, so I'd expect them to just be cheap semiconductor lasers.

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I think the difference is partly because the human eye is more sensitive to green wavelengths than red.

 

This is very true.

[coke]

wait a sec... so these laser diodes work like leds without requiring a lens? by using some similar pump mechanism (simulated emission)?

[swansont]

I think the difference is partly because the human eye is more sensitive to green wavelengths than red.

 

Bingo! It can easily be a factor of 10 difference, depending on the actual wavelengths involved.

 

Beware: some green lasers will say that it's as bright as a 50 mW red laser, and end up being a 5 mW laser (or some other values with a similar scale disparity), because of the sensitivity difference. It's not so common anymore, but was standard practice a few years ago when the green lasers had just come out, and more than a few mW was tough to get.

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wait a sec... so these laser diodes work like leds without requiring a lens? by using some similar pump mechanism (simulated emission)?

 

Laser diodes use lenses, but they are built into the casing when you buy a laser pointer. Bare diodes have horrendous diffraction and astigmatism — the gain medium is rectangular, and the light diverges but has a different focal point for the x and y directions. The outgoing beam is elliptical but the long direction changes by 90º near the laser. It's a real pain to get a good beam from them — you end up throwing away a lot of light. Good thing they're relatively cheap, as lab lasers go.