"Invisible hand" (to a laser)

[5614]

well to be honest i didnt know what to call this thread!!!

 

if i take my thumb or any finger (average sized) and place my laser pen on the other side of it i can see the red light through my finger... not as powerful as if it there was no finger... but significantly, or enough to clearly notice it.

 

as i move the laser down my finer it becomes fainter (the top of my finger must be slightly thinner) and even through my palm i can (a very tiny bit) see it... however my arm and pressumably most of the rest of my body is too thick to let the light through (no rude comments about certain areas of the body please )

 

so my question is how are the photons from the laser getting through?

 

are they just travelling through the atoms? becuase afterall atoms are mostly a vacum through which a photon could pass.

[Cap'n Refsmmat]

If you notice, the light dissipates instead of going shooting straight through your hand. My assumption is that the cells normally only let a tiny amount of light through, but when the light is powerful enough (laser) some gets through and causes the effect.

[ydoaPs]

i thought it was the thing where the photons cause the electrons to go up a level. and they later emit a photon.

[Cap'n Refsmmat]

Yes, but normally your hand is opaque.

Perhaps the atoms in the hand will only emit light when the stimulation is powerful enough, as I said.

[5614]

one sec, the atoms in your hand are always emitting photons, that's how you see your hands... its just normally the light is reflected off your hands and doesnt go through em!

 

i thought it might be the photons travelling between the atoms, because as you know, atoms are mostly a vacum.

[ydoaPs]

there is a difference between emitting and reflecting.

[5614]

indeed but a photon must be emitted from the atoms in your hand for it to be reflected... i know what you mean, but technically even when your hand is reflecting light it is emitting photons.

 

also nice sig, but you need the saying that triggered his response, i remember it, but others may not... it was funny!

[swansont]

Each atom or molecule has a certain probabilityof absorbing a photon. Typically this results in an exponential decay of the intensity with distance. the laser is distinctive and bright enough that you can see the light, plus there is re-emission (as was mentioned) when a molecule de-excites. It works in the hands because they are relatively thin.

 

You'll get the same effect with a flashlight, but the light will be red.

[YT2095]

a bit of a give-away that this is the case is that you can`t make out any bones properly ether, only a slightly Less Bright area.

[5614]

how does an atom know at what angle to emit it's photon? remebering that the angle of incidence = the angle of reflection...

 

is the light i am seeing light which has travelled directly through my hand or is it light which has come in at a weird angle and been reemitted through the other side of my finger and happened not to bounce into any other atoms?

 

also if a photon hits an atom it excites the atom and electrons jump up to the next 'level' i know all of that, but if a photon hits an electron i can see the energy transfer, but what if a photon hits the nucleus, how does the energy get to the electrons?

[YT2095]

in this instance it`s little to nothing to do with that, it`s more akin to trying to look through a frosted glass bathroom window and identifying who`s outside

 

the photons scatter (diffuse).

[swansont]

how does an atom know at what angle to emit it's photon? remebering that the angle of incidence = the angle of reflection...

 

is the light i am seeing light which has travelled directly through my hand or is it light which has come in at a weird angle and been reemitted through the other side of my finger and happened not to bounce into any other atoms?

 

also if a photon hits an atom it excites the atom and electrons jump up to the next 'level' i know all of that' date=' but if a photon hits an electron i can see the energy transfer, but what if a photon hits the nucleus, how does the energy get to the electrons?[/quote']

 

For reflection, there are some conservation laws from QED that apply.

 

As far as what the photon "hits," remember that light has a spatial extent - you really can't localize it to smaller than a wavelength. A photon is absorbed by the atom, not specifically the electron. (and free electrons can only scatter photons; they can never absorb them)