1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

How many times does a atom excitate when light shines onto it?

  1. Oct 10, 2014 #1
    Noob question ahead.

    So basically I'm reading about atoms and photons in my textbook at the moment and I came across excitation and why atoms emit photos with certain wavelengths when hit by white light. The claptrap doesn't really matter, what I'm confused and curious about is how many times does an atom "excite" every second when light shines onto it.

    I've understood that when photos hit an atom, i.e. hydrogen the electrons around the nucleus gets excited and jumps up one energy level(orbital), and when they "fall" down again, the difference in energy is what gets emitted as a photon. This is the part that I am curious about. If a 1000 photos hit an atom a second does that basically mean that the single atom gets excited 1000 times and emits 1000 photos with the wavelength that accompanies that change in energy.

    Please help, thank you.
  2. jcsd
  3. Oct 10, 2014 #2
    Try to estimate how many photons you shine onto your sample per, say, 1 second.

    Then try to estimate how many atoms the photons are illuminating.

    The ratio is the maximum average number of excitations. There will be a statistical distribution, of course, so some atoms get excited more often than others. But at least this gives you a ballpark number. Also, this assumes that every photon that his the sample actually excites an atom. This is not always the case.

    The fewer atoms you have, e.g. if you decrease the pressure in a gas, the smaller is the chance that a photon "hits" an atom and excites it. Most photons will simply be transmitted without interacting with the atoms.
  4. Oct 14, 2014 #3
    Sorry for the very late response, but forgot about this and been busy. If you do have the patience though, could you please link me to the formula for calculating photos per second. In my book, they expect you to manipulate and use the wave formula together with the formula for wavelength energy formula or something, here; E = h * f
    Nowhere in the book does it say how to do this, but I tried manipulating the formulas a tiny bit and noticed that it has something to do with the planck constant, but my answers seem rather inconsistent, but please correct me thank you.
  5. Oct 14, 2014 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    The answer to your question depends on at least two things:

    1. How likely the atom is to be excited by a passing photon.
    2. How long it takes for the excited state to decay.

    If your atom simply doesn't have time to decay from its excited state before the next photon "hits", then it won't absorb the photon. The amount of time an atom remains in an excited state can be very different for different states, so there is no single answer to your question.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook