Light and LASERs

[Rasori]

I heard it said somewhere that light is both a particle and a wave.

 

Yeah, okay. I'm in eighth grade, this tells me very little

 

So, light is a particle, because it has to be otherwise it can't travel through space. So then, why does it have wave properties? Why can't it just be a particle? (I'm sure this is probably something obvious- it needs to cover too much area for a few particles to work on it, or something.)

 

Also, when it hits something that isn't a vaccuum, like our atmosphere, does the particle dissipate, or what? I'm assuming that's the point when it switches to waves, but why and how?

 

Now onto the other topic- lasers. Lasers are still light, so it still consists of particles and waves. The main difference: the light stays in a straight line. Other than this, what are the other major differences?

 

I'm sure that I, being the uneducated fool that I am, will come up with more stupid questions, but this should be enough for now.

[YT2095]

Laser light is is colminated and also ploarised as an effect. the the direction of photons emited is coherant and orderly as opposed to ordinary light that is generaly scattered (from a photinic point of veiw).

[Cap'n Refsmmat]

YT2095 said in post # :

Laser light is is colminated and also ploarised as an effect. the the direction of photons emited is coherant and orderly as opposed to ordinary light that is generaly scattered (from a photinic point of veiw).

That makes it sound more like you just point and polarize. Actually, lasers light emits all the same frequency of photons. Not a bunch of different frequencies and colors, like white light. It's pure, so it hardly scatters.

By the way, it's still a wave in a vaccuum. It doesn't change.

[-Demosthenes-]

Wait, how could it be a wave in a vacuum?? Waves have tomove through a medium right??

[Cap'n Refsmmat]

No. The photon is the medium. Somehow. (I'm not the physicist here)

[Rasori]

Hmm...

So it's a wave on its own particles...

 

Interesting, to say the least. So that means that if you somehow removed the photon that that wave of light would cease to exist? For example, if there was a laser in space and you were to remove the photons from it's beam, the laser would cease to continue onward?

[Crash]

NO, it behaves like waves in wave experiments, and it behaves like a particle in particle experiments. Its still considered both at the same time but not like how you put it.....as for the idea of having to have a medium thats rubbish as long as your willing to abandon half of newtonian mechanics in order for the more new rewritten concept

BTW read "brief history of time".........it helps

[Rasori]

Brief history of time?

 

Hmm... sorry, sounds like an oxymoron, since time's been around for billions of years. Probably explains why I hadn't read it before.

 

So it's both, but even in a vacuum it's still a wave, and not on its own particles... hmm.

 

Or were we both misunderstood? I said:

"Interesting, to say the least. So that means that if you somehow removed the photon that that wave of light would cease to exist? For example, if there was a laser in space and you were to remove the photons from it's beam, the laser would cease to continue onward? "

 

But I meant more like:

Interesting, to say the least. So that means that if you somehow removed the photon in a wave of light in space then that wave of light would cease to exist? For example, if there was a laser in space and you were to remove the photons from it's beam, the laser would cease to continue onward?

 

Probably doesn't make a difference, but just wondering.

[Crash]

"Brief history of time" is by stephen hawking, and is very helpfull

[Skye]

Rasori, I think the idea is that light has the properties of both a particle and a wave, not that it consists of a seperate wave or particle, they are one and the same thing.

[Radical Edward]

photons (everything actually) have both particle, and wave like properties. One can say thay are like a wave that acts in quanta (discrete interactions) - it doesn't really swap between one and the other as implied above. This is of course very strange and unfamiliar to us, but then all of quantum mechanics is.

 

Onto Lasers. The acronym is: "Light Amplification by the Simulating Emission of Radiation"

 

Essentially there are 2 ways that an excited electron can decay to it's ground state: one is "spontaneous emission" which is effectively randome, and the other is "stimulated emission" which is when the intensity of the light field is so strong, that the electron is forced to drop into the ground state and emit a photon. This causes the light to be both amplified (since the strong field forces more photons to be emitted, which makes the field stronger and so on. This effect is achieved in a cavity resonator - basically a lasing medium between two mirrors.

 

The most notable quality of laser light is that it is coherent; all the waves are in phase with one another. Lasers are often near-monochromatic, but not always, and often, but not always polarized. They are generally fairly well collimated too, though only to the diffraction limit - so tiny lasers will spread alot faster than big ones.

 

There are a few other clever things about lasers, for example one can only lase a medium when one achieves population inversion - that means there are more excited electrons than unexcited ones. This is thermodynamically troublesome, since conventionally you can only have an even split between the two (in steady state) though you can play tricks by using a multilevel laser.

 

I appreciate alot of this will be a bit complicated, so if you have more questions on the terms I used or anything, feel free to ask.

[Radical Edward]

Cap'n Refsmmat said in post # :

That makes it sound more like you just point and polarize. Actually, lasers light emits all the same frequency of photons. Not a bunch of different frequencies and colors, like white light. It's pure, so it hardly scatters.

By the way, it's still a wave in a vaccuum. It doesn't change.

 

not really, there are many broadband laser sources too, such as dye lasers (really nasty carcinogenic stuff some of them, comes in big cans that say do not eat on the side), NdYAG (Neodinium Yttrium Aluminium Garnet) lasers and so on. These are extremely important in short pulse lasers where you need a wide frequency range in order to compress the pulse. They are also used in tunable lasers too, which are often seen in those big outdoor laser shows. The collimation of a laser is more to do with how the laser is formed: essentially to amplify the light significantly you need to mirrors facing one another (the mirrors are generally concave, but you can get away with other mirror arrangements if you are careful and know what you are doing) which means that the beam has to bounce back and forth inside the cavity for a long time (well, a long time for the light anyway) before it is emitted.

[Rasori]

Wow...

 

So a particle that acts like a wave... I'm just gonna stop there, because I get it but I still want to repeat it in disbelief. But you all wouldn't want to lie to me.

 

Is there a reason why sometimes you can see the beam of the laser and other times you can't? I've had a laser pointer (granted, one of those like 5 dollar ones, but still a laser pointer) and you could always see the point that it makes. But sometimes, the line between the point and the emitter is invisible. Sometimes you can even see part of it and a little later you can't. Is it just the angle of it, or what? Or am I just imagining things?

 

Otherwise, I think I understand what I first asked about.

[blike]

Rasori, check out the "Young's Double Slit" experiment. It's a fabulous demonstration of light's wave-like properties (and some other strange properties).

[Cap'n Refsmmat]

The reason you can normally see the beam is the dust in the air. Normal lasers beams shouldn't, because it's not supposed to spread out.

[Crash]

i once got to see an Argon ion laser (at otago university), it was awesome the guy using it there had heaps of scares on his hand (for obvious reasons).....

Is the "double slit" thing demonstrating the diffraction grating idea?

[Rasori]

So it's just bouncing off the dust particles, basically?

 

Have any major experiments been performed regarding lasers in a vacuum? Particularly the one we all lovingly call 'space' or some derivation thereof? Just curious on how the perform in extreme temperatures and extreme... uh... nothingness...

[Crash]

space is far from a vacuum or "extreme nothingness"

[Rasori]

Beside the point.

 

But the majority of it... or at least quite a bit of it, is a vacuum.

[Crash]

i dont know of any laser experiments using lasers in "space" but several have been done in vacuums, or particularily with EXTREMELY small amounts of atoms in a vacuum and hitting them with lasers

[superchump]

They bounce a laser off the Moon to measure its distance from the earth. A mirror was left there by one of the Apollo missions and is struck by a very coherent beam as so it doesn't expand too much during its short travel time.

[Radical Edward]

young's slits is quite easy to do at home - here's how you do it.

 

take a piece of glass, like a microscope slide or something, and then put it over a candle flame (a reasonable distance so that it doesn't crack or anything) to blacken it. Then scrape away a very thin line. shine a laser through it, and project it against a wall, you should see a diffraction pattern, that is, a bright central line, and just next to it a darker kine, and next to that a darker line still.

 

now if you scratch a second very thin line close to that one, you see the same pattern as before, but now you see nodes of light imposed on that one (like you are chopping the original pattern into slices). Looking at the distance between the bands of light, you can work out the distance between the slits if you know the wavelength of the light ( a standard red laser that you can buy is usually about 600nm or so) . I will leave this to you - just do it with basic trigonometry.

 

Young's slits is just the 2 slit version of a diffraction grating, which is the same thing, but with an awful lot of slits.

[Radical Edward]

superchump said in post # :

They bounce a laser off the Moon to measure its distance from the earth. A mirror was left there by one of the Apollo missions and is struck by a very coherent beam as so it doesn't expand too much during its short travel time.

 

actually it does expand a heck of a lot. The amount of light they pick up is absolutely miniscule. The reason for this is the diffraction limit. I won't go into this in depth, but basically, for an aperture of diameter D, the maximum angle (in radians) that it can resolve is approximately

 

Theta = 1.22(lambda)/D

 

where theta is the angle, lambda is the wavelength of the light and D is the diameter.

 

now the angle seems pretty small, but when you plug that angle into a triangle for which the adjacent side to the angle is the earth-moon distance, you will find that the opposite side (i.e. the diameter on the moon) is absolutely huge, and even bigger on a round-trip back to the earth.

 

 

I did this for an aperture of 1m in diameter (quite large for a laser) and a wavelength of 550nm, and by time it gets to the moon, it is about 13km across.

[Crash]

Yea i can remember doing this in High school, i could never get the lines right, as we used graphite insteasd of soot, and it would crack etc...

What size is this mirror on the moon?

[Radical Edward]

I just remembered, you can do the diffraction slit thing with a CD and work out the amount of data a CD can hold

 

I am not sure how big the mirrors on the moon are. probably a metre or so (so you have to treat them as the aperture now)