1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Why do photons pass through particles with a delay

  1. Apr 24, 2017 #1
    hi everyone.
    i'm only 16 so please beer that in mind.
    I've been doing some reading around and have come across the idea that light slows down in an dense medium as a result of the photons moving tough other particals with a slight delay however they retain all factors about them selves apron leaving the partical its traveling through. my question s what exactly is going on, how does this happen and why does it happen?
    many thanks for any answers in advance.
    Ewen shackel
  2. jcsd
  3. Apr 24, 2017 #2


    Staff: Mentor

    Good for you, thinking about such things at age 16. I think the quote below is what you are asking about. The entire wiki article may be of interest to you.

    By the way, on PF you should just ask your question. It is against the rules to express your personal theory.

  4. Apr 24, 2017 #3
    sorry i assent expressing any theory that i thought of as such i was just asking about what i had been reading up about and not knowing the name had tried to describe it as best as i could. but my apologies and ill make sure to mention i am not posing a theory next time.
    thank you
  5. Apr 24, 2017 #4


    User Avatar
    Science Advisor
    Gold Member

    It is a bit complicated. To give a proper treatment we would want to look at the quantum mechanics of light interacting with matter but I'll try a bit of a rough explanation. Firstly since light is quantum mechanical at its root either describing it as a wave or describing it as a particle are neither correct. But describing it as a wave is better for some phenomena and describing it as a particle is better for others. Here we should avoid the particle description and use a wave description. So let's not speak of photons but rather light as electromagnetic waves.

    Since all matter is made up of charged particles (protons and electrons) even if the total charge is neutral there will be some interaction with light as an electromagnetic wave. As such it is typically the case that when light waves hit matter it will bounce off randomly and impart random energy into the matter. You see and feel this when sunlight hits your arm and scatters in all directions including your eyes, and some of it warms your skin.

    What is remarkable is that some substances do allow light to travel through and mostly come out the other side just as it entered. Such transparent materials are typically have their atoms tightly bound together and reasonably uniform in density.

    What happens as light travels through these materials is that a small part of the light is absorbed by the matter and re-emitted. This absorption-emission process produces a phase shift corresponding to a slight delay.

    If you still want to think in terms of photons, imagine an army of photonic soldiers running in formation. If they hit a the woods they would smack into trees and get disoriented and scattered. But if they are just running in a field with occasional dirt piles and holes, you will see the soldier occasionally trip or snag a toe. They will keep running but be slightly behind and out of step. But since the obstacles are regularly spaced, eventually all of the soldiers have tripped about the same number of times and be more or less in step and in formation when they leave the field and are back on the road.

    Another example which is closer to the actual case is to think of what happens with radio waves. If you have a series of grounded antennas scattered around and a radio wave comes by it will induce currents in the antennae which will then re-radiate. Depending on how the antennas are tuned and spaced the incoming radio wave or it can be effectively slowed over a distance as it gets continuously re-radiated. In fact the parallel metal conductors you see on a classic Yagi VHF antenna are in this way a sort of lens focusing the incoming radio wave onto the active dipole (others act as a reflector as well).

    The key issue here is "elastic" interaction in that all the energy and momentum of the light must end up only being temporarily absorbed. Recall that once you give the light's momentum vector, energy and polarization, you specify the light up to a factor of its phase.

    Finally there is a law of large numbers here as well. As you add up very very many very small random effects the net result will have a very small standard deviation... there will be very little deviation away from the net average effect. So all these little partial absorption re-emission effects tend to add up to a rather precise amount of delay.
  6. Apr 24, 2017 #5
    The problem with this analogy is the scale. The soldiers would be orders of magnitude larger than the trees.
  7. Apr 24, 2017 #6


    User Avatar
    Science Advisor
    Gold Member

    Yea, I know... I didn't want to get into too much detail, (I am long winded enough as it is). Actually the analogy needs to be better to first express coherent reflection which is another absorption re-emission process but with stronger coupling. Then most materials are aggregates of absorption (slogging through the mud) reflection and transmission with refractive scattering. But if you think of the trees and spaces between as domains in an aggregate material then the scale is not too far off.
  8. Apr 26, 2017 #7


    User Avatar
    Gold Member

    The following article says that glass is transparent because light photons do not have sufficient energy to raise an electron to a higher energy band.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted