Do Photons Follow the Rules of Mechanics for Collisions?

In summary: So, in summary, photons can transfer their momentum to other particles, depending on the energy involved.
  • #1
Jeebus
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If you do some simple manipulations of the equations E=hf, E=1/2mv^2, P=mv, and p=h/<lambda>, one can derive that the momentum is h/<lambda> or that it is h/(2<lambda>). I know that photons do not always follow normal mechanics, but do they follow the usual rules for mechanics for collisions?

I have heard of numerous situations in which photons have transferred their momentum to other particles or even larger masses, but it is unclear to me how exactly this works. It seems that one could build a device which would have an efficiency greater than 100 percent. In other words, it would not follow the law of conservation of energy.

Why can this not be done?

Is it because the photons transfer energy through various mechanisms, depending on energy?
 
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  • #2
The momentum of a photon is h/&lambda; and yes they can be involved in collisions with free electrons (compton scattering), where their wavelength is depednet on their angle of scattering. Photons can be absorbed by electrons in bound states too.
 
  • #3
Originally posted by Jeebus
If you do some simple manipulations of the equations E=hf, E=1/2mv^2, P=mv, and p=h/<lambda>, one can derive that the momentum is h/<lambda> or that it is h/(2<lambda>).

The formulas you used are non-relativistic and do not apply to photons. But the equation p = h/&lambda; is correct.


I know that photons do not always follow normal mechanics, but do they follow the usual rules for mechanics for collisions?

They obey the conservation laws, if that's what you mean.


I have heard of numerous situations in which photons have transferred their momentum to other particles or even larger masses, but it is unclear to me how exactly this works.

Pretty much the same way any other particle transfers momentum.


It seems that one could build a device which would have an efficiency greater than 100 percent. In other words, it would not follow the law of conservation of energy.

Why?
 
  • #4
Originally posted by Jeebus
I have heard of numerous situations in which photons have transferred their momentum to other particles or even larger masses

Is this why you say that...

...It seems that one could build a device which would have an efficiency greater than 100 percent.

??

The energy of a particle is given by

E2 = m2c4 + p2c2

A photon, being massless, has an energy E = pc, which can be quite substantial, and it definitely can be transferred to massive particles.
 
  • #5
a photon transfers its momentum or used to increase orbit enegy? 2 different cases or what? Exactly how does it transfer momemtum
 
  • #6
In lower energy photons it usually results in heat. Higher energy photons, in about the UV range or greater, can ionize atoms by ejecting electrons. On the extreme end the photons can undergo Pair Production, where a photon interacts with a nucleus and its energy is used to create pairs of particles such as an electron and positron.
 

1. Do photons have mass?

No, photons do not have mass. They are considered to be massless particles and therefore do not follow the rules of mechanics for collisions in the traditional sense.

2. How do photons interact with matter?

Photons interact with matter through a process called scattering, where the photon's energy and momentum are transferred to the particles in the matter. This can result in absorption, reflection, or transmission of the photon.

3. Can photons collide with each other?

No, photons cannot collide with each other in the traditional sense. They can interact and exchange energy through a process called photon-photon scattering, but they do not behave like classical particles in a collision.

4. How do the rules of mechanics for collisions apply to photons?

The rules of mechanics for collisions, such as conservation of momentum and energy, do not directly apply to photons since they do not have mass. However, these principles can be applied to the particles in matter that interact with photons during scattering events.

5. Can photons change direction during a collision?

Yes, photons can change direction during a collision with matter. This is due to the transfer of momentum from the photon to the particles in the matter, resulting in a change in the photon's direction. However, photons do not undergo a change in velocity or speed during these interactions.

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