if photons have no mass, why do they move slower in the atmosphere..?

Question

my guess is that they (photons) must move around any particles they incounter in an atmosphere. any ideas out there? thanks

Answer 1

It's because the photons are absorbed and then re-emitted by the molecules in the air. It takes a certain (small) amount of time between absorption and re-emission, and this causes a delay. So, just as your progress will be slowed down if you have to stop at a lot of traffic lights, so the progress of the collection of photons is slowed down by being absorbed. When they travel BETWEEN one molecule and the next, they are still traveling at speed "c".

Answer 2

It is because of the wave nature of light. Waves behave differently when they travel through different mediums. It can also be observed with earthquakes as some waves of energy travel at different speeds through different layers of rock with different composition and density.

The speed of light in vacuum is the fastest anything can travel. But the speed of light is different in every medium. It is slower in air, slower still in water and even slower in glass. This is just the nature of light. The above suggestions, that light is absorbed and re-emitted, is only partly true. It is why the sky is blue but it does not explain why light traveling through a prism travels at the same speed before and after it hits the prism while the speed inside the glass is two thirds the speed of the outside as if the photons magically accelerates. That explanation would also yield photons of quite random speeds as some photons that miss air molecules would travel faster. These things have not been observed. And so the above explanations are wrong.

The phenomena, with the speed of light being different in all mediums, is why we have refraction. And lenses and telescopes etc.

Answer 3

Light (photons) moves slower in air because molecules in the air will absorb and then re-emitted the photons, slowing them down a little each time they are absorbed and re-emitted. Think of it like hitting a stop sign every so often on the freeway. This would slow you down during a long trip.

Answer 4

Light waves (and all electromagnetic radiation) exhibit properties of both matter and waves, so it has some weird effects.

But in terms of refraction/diffraction or the slowing down, EM waves interact with other atoms and cause them to vibrate. They cause changes in energy states in the electrons, which emit photons because of this, so in turn they create their own EM waves. Even though they are smaller, can alter the original EM light waves through wave interference.

[Edit]
Rick B is right with the stoplight analogy, which is the main reason it slows down. However wave interference also plays a part.

Answer 5

still get affected by the fields set up by the electrons etc the photons pass by