Physics question

[Goosemaster]

basically, in terms of Photons, light behaves like a particle, right?

How is it that light gets absorbed into absorbant materials and is released as heat?

For example, lets look at heat. Heat is a bombarment of collissions between molecules in a substance that results in increased enerygy and energy given off, it is not?

So how is it that light, which has no mass (photons) bombards or collides with actual molecules and excites them?

My friend explained that photons still have momentum. Tothat I responded that the definition of momentum is p=mv or p=m(dv/dt) but that would not seem to apply.

What I hypothesized (with my crude knowledge of phsics...I am in Eng, Physics 1):

Assuming that light exhibited particulate properties, perhaps the universal gravitational constant might apply, as in it might attract objects, even given its appearantly nonexistant mass. Perhaps this gravity could in fact act as a force over a distance, to act upon molecules and create some type of collissions...


By then the discussion trailed off, and he jsut told me to look up wave mechanics and such.so I as k you people, what should I look into first?

thanks

 

[Goosemaster]

Keep in mind that it is 3:36am, I am going to bed, I have physics @ 8am, and I might not be back for a while to check your resposnses so don't start thinkign that I am ignoring you

later

 

[n0cmonkey]

Ask your teacher at 8:01am. 😛

 

[Hankysmoo]

LOL, good answer

 

[Goosemaster]

Originally posted by: n0cmonkey
Ask your teacher at 8:01am. 😛

Last time just looked at me was like..."yeah right."

He avid proponent of "not wasting time with stupid questions."

The guy is smart and gnereally nice, but he doesn;t like answering [appearantly far out ] questions like this

 

[iwantanewcomputer]

the photon hits an electron orbiting an atom of the material. the energy of the photon is converted to kinetic energy of the electron.

the photon does have momentum but it is defined by planks constant. basically you can use the energy of the photon, E=h* frequency, to find the gain in energy of the electron.

the influence of gravity is negligable on this scale. it is only apparent when light bends around a star or black hole.

this is all basic quantum mechanics

 

[Ophir]

Answer

Basically, IIRC from QM I took 3yrs ago, due to relativity photons have a rest mass of zero. However, since they travel at the speed of light, things get complicated. At the speed of light they have an "apparent mass" which appears out of the quantum mechanical and relativistic relations: E=h*nu=m*c^2. Therefore, m(apparent)=(h*nu)/c^2. AFAIK this apparent mass doesn't really mean anything other than to make the equations work and satisfy the Newtonian thinking types out there. Most of this thinking goes out the window when dealing on a quantum level (or else you'll end up pulling your hair out). Photon energy and momentum are just accepted and mass is generally ignored.

After this it's basically a simple question of energy transfer.

Have fun in QM and SS EE :evil:
:beer: (You're gonna need it)

Don't quote me on this, though. It's only IIRC.

 

[Ophir]

Originally posted by: Goosemaster

Originally posted by: n0cmonkey
Ask your teacher at 8:01am. 😛

Last time just looked at me was like..."yeah right."

He avid proponent of "not wasting time with stupid questions."

The guy is smart and gnereally nice, but he doesn;t like answering [appearantly far out ] questions like this

This question is not "far out" it's just well out of the scope of a physics 1 class. To explain it correctly he would have to explain relativity and basic QM. He surely knows full well what the answer is, he just doesn't want to spend an hour explaining it.

You WILL deal with this question in the future if you do an physics or EE (or EE type) major. It's a pretty basic and essential question and is usually dealt with by the 2nd week of quantum mechanics.

 

[Goosemaster]

cool, so with photons d(mv)/dt applies?😀


<--goes off to read some QM books

Thanks

 

[Beller0ph1]

This is a simple blackbody radiation problem. Heat is just infrared electromagnetic radiation. When light strikes an object and is absorbed, the energy from the photon excited electrons of the object. They jump to higher levels, relax a bit, then emit light at a lower wavelenght than whatever hit it. When you heat objects, the start releasing energy in the form of electromagnetic radiation (IR light), but when they are heated up enough, they begin to emit shorter wavelengths, hence they glow.

 

[Gibson486]

My friend explained that photons still have momentum. Tothat I responded that the definition of momentum is p=mv or p=m(dv/dt) but that would not seem to apply.

This goes well beyond physics 1, and since you are going into something with particles and speed of light Newton's law may need modification.

Also, your equation for momentum does not seem right. How can it be equal to Force also? Perhaps you meant to say p=m(dv/dt)*t

 

[edro]

Quick Google Search result:

Question - In your archive question # 24 you state that lasers have
a small amount of recoil due to the momentum of the photons leaving it.
But if a photon has momentum, doesn't it also have to have mass?
If so, then since it is traveling at light speed, wouldn't it's mass would
have to be multiplied by infinity? But this couldnt happen, because the
universe would collapse every time a photon was generated.

Also, why does magnetic field strength decrease much faster than the
inverse square of the distance like gravity? Is it because the force lines
have to curve back to the source? Is magnetism also carried by an
"imaginary" particle?
------------------------------------------
This is several questions:
1. It a result of the theory of relativity that a photon has momentum, but
have zero mass. One of the worrisome aspects of relativity and quantum
mechanics is that we must abandon our graphic notions of Newtonian
mechanics, and also some of the notions of classical electromagnetic theory
of Maxwell. The problem you are asking about is only one of several notions
that has to be abandoned. In addition, there is the "fact" that the photon
has angular momentum -- but from the classical picture, "something" has to
be spinning. What's spinning? And an electron orbiting a proton in the
hydrogen atom should spiral into the proton emitting electromagnetic
radiation, but it doesn't. Why not? The "why not" to all the above is that
our classical picture just does not correspond to observation, so we are
forced to reject our "picture" and adhere to the "observation" and not the
other way around. Richard Feynman
discusses the momentum of a photon in Vol. I - 34 - 10 of his Lectures on
Physics.

2. The force generated by a magnetic field does not obey an inverse square
law -- a fact. It obeys the force law associated with a dipole. When you ask
for a "Why?" modern physics is at somewhat of a loss because there really
isn't an easy answer. It doesn't (at least at our current state of
knowledge) and we have to live with that. I don't know that there is any
answer to the question "Why?" I believe it is important to always add, "At
least at our current state of knowledge." provision, because a few years ago
no one would have thought it possible to bring a photon to a dead stop, but
it has been done, and done so rather convincingly.




Interesting... Even the greatest physicists in the world don't have a "gut" answer for it 🙂

 

[Ophir]

Originally posted by: Goosemaster
cool, so with photons d(mv)/dt applies?😀


<--goes off to read some QM books

Thanks

Umm... yeah, but in a really screwy way. In quantum mechanics, the same basic laws work - conservation of energy, conservation of momentum - but everything is different. It's tough to explain.

Are you doing this as a class project, or something? Because if it's just to fulfill your curiosity, it is NOT worth your time to go through QM books. I guarantee it'll just blow your mind and confuse you more. At the atomic level, Newtonian Mechanics breaks down and needs many modifiers to approximate what's going on. Hell even quantum mechanics guys don't understand it.

Trying to apply your very basic understanding of Newtonian Mechanics and applying it to QM in NOT going to work.

Just take the fact that photons have energy (and momentum) which excites electrons and stimulates the emission of heat (the basic black body problem), as fact.

You will deal with this problem soon enough, and the subsequent work will make you miserable. Go out and enjoy college life, don't waste it trying to be the wunderkind who knows QM.