# Does photon also comes at rest after sometime while travelling through a medium?

If you fire a bullet in air, after sometime it comes to rest because of air resistance like forces. but what about photons? If they are also particles, then the same thing should happen with them. Does not matter how long it takes but they should come to rest as well while travelling through a medium. It happens or not?

A photon is a particle but it has no mass, that is, mass is zero. Because it's mass is zero, it must always travel at the speed of light: $299,792,458$ m/s. Thus, it is never at rest -- Never.

However, this does not mean that a photon is constantly moving forever. Photons interact with other particles that just get into their way. For example, a photon can strike an electron in an atom be completely absorbed with the result that the electron is knocked into a higher energy level representative of the energy given to it by the photon. The photon itself at that point ceases to exist.

But, usually that excited electron is not stable at that higher energy and it will fall back down to the lower energy level where it was before and another photon of energy will be released and this new photon then continues traveling at the speed of light until it interacts with yet another particle.

• OK. Does that mean like a bullet or other massive objects are affected by friction and air resistance, photon is affected by charged particles? – sonal Jun 11 '17 at 6:19
• Yes, photons only interact with electrically charged particles. – WAH Jun 11 '17 at 6:21
• @WAH How do you think a neutron gain kinetic energy? – HolgerFiedler Jun 11 '17 at 7:10
• @HolgerFiedler: in what situation? Like in beta decay, in which case the weak force is mediating? – WAH Jun 11 '17 at 14:59
• @WAH A neutron can gain kinetic energy simply if a photon hits a neutron. Isn't this an interaction of a photon with an uncharged particle? – HolgerFiedler Jun 11 '17 at 16:32

If you fire a bullet in air, after sometime it comes to rest because of air resistance like forces.

A bullet is a particle defined in classical mechanics, , it has a mass within a volume in space and at time t its center of mass has a well defined trajectory.

but what about photons? If they are also particles, then the same thing should happen with them.

Photons are quantum mechanical entities, defined as particles in the elementary particle table, with zero mass, and obeying special relativity equations..

There is a huge difference between the two concepts, because elementary particles even though they have a well defined mass are point particles. The term "particles is used because trajectories can be calculated, but the physical behavior is controlled by the probabilistic equations of quantum mechanics..

Light is built up out of zillions of photons, but photons are not light. Because of their zero mass they can never be at rest and always travel with velocity c. Have a look at this answer of mine to a related question.

Light can be slowed down and even stopped , but that is another story.