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!

Power of laser vs Power of X-ray

  1. Aug 23, 2010 #1
    I wonder how to compare the power of a laser and an X-ray. For example 1mW power of a laser hitting an object is comparable to the an X-ray generating 1mW of power?
    Thank you.
  2. jcsd
  3. Aug 23, 2010 #2
    I suppose you're interested in how much is absorbed by the object in each case? It depends on the object. You're comparing apples to oranges. Very different effects.
  4. Aug 23, 2010 #3
    Yes, the amount of absorbed energy is what I am looking for. So, which properties of the material I should check to see the absorption for these two sources?
  5. Aug 23, 2010 #4


    User Avatar
    Science Advisor

    Would it be disappointingly simple if I answered "the absorption spectrum"?
  6. Aug 23, 2010 #5


    User Avatar

    Staff: Mentor

    At face value, the question sounds like the old riddle: what weighs more, a ton of feathers or a ton of bricks...?
  7. Aug 23, 2010 #6
    I am sorry if my background is not good.
    Is it right to say photons produce laser, and electrons are the origin of x-ray?
    If so, the absorption spectrum for these two is different for a given material?
    Thank you.
  8. Aug 23, 2010 #7


    User Avatar
    Science Advisor

    X-rays are one area of the electromagnetic spectrum, a designation of a particular set of wavelengths of light (about 0.01 to 10 nm), laser light is usually in the visible range or infrared, so around 100-1000 nm. (look up the electromagnetic spectrum for more on that) Photons are "particles" of light (or more correctly, electromagnetic radiation). So you have x-ray photons and visible photons, depending on the wavelength.

    The photons in a laser beam are produced by the electrons in a material (such as a gas, for a gas laser) moving from a higher energy level to a lower one within the atoms/molecules, giving off the extra energy as a photon. The photon's wavelength is a measure of how much energy the photon has. (lower wavelength means higher energy and vice-versa). The more energetic x-rays are given off by nuclear decay (gamma rays) and by very high energy electrons, such as those accelerated by a huge voltage difference (a 'cathode tube').

    The absorption spectrum is very different, because very different things happen. When a photon of visible light hits a material and it is absorbed (if it is absorbed), the electrons go to a higher energy state within the atoms/molecules (i.e. the same kind of thing that happens when it was emitted in the laser, but in reverse). After absorbing it, the electrons eventually go back to their original states, and the photon's energy may become heat, or it might be re-emitted at the same or at a different, lower, wavelength (fluorescence).

    When an x-ray is absorbed, on the other hand, the electrons gain so much energy that they leave the atom/molecule entirely. They get 'kicked out of orbit' so to speak. The atom/molecule, having lost a negatively-charged electron, ends up with a positive net charge, making it an ion. For this reason, x-rays and gamma rays are also called 'ionizing radiation'. The electrons eventually make their way back to the atoms/molecules, and heat and radiation is given off in the process. (for instance, some substances visibly glow when exposed to x-rays)

    This is why x-rays are dangerous. They have so much energy that they can easily break chemical bonds, such as in our DNA - which causes cancer. Visible light, on the other hand, rarely changes chemistry (when it does it's called a photochemical reaction). So that's what those two forms of radiation do. Although there are other areas of the spectrum as well, which cause other physical phenomena.

    Then there's the matter of absorption. It's not very easy to predict the probability of absorption. There's no simple relationship to any everyday property. Basically you have to know how the electrons move around the atoms/molecules to know what their energy states are. Once you know that, you can figure out the probabilities of them absorbing radiation of a particular wavelength. But it's a difficult problem, since electrons repel each other, so the motion of each electron in an atom depends on every other electron in the atom, and it gets mathematically very complicated. Even with computers it's difficult.

    But we can still immediately say some things about the different kinds of radiation. To begin with, you likely already know that neither you, or most everyday objects are transparent, in other words, they absorb and/or reflect most visual light that hits them, whereas x-rays mostly go straight through almost everything.
  9. Aug 24, 2010 #8
    Thank you very much for your time. It was very informative.
  10. Aug 25, 2010 #9


    User Avatar
    Science Advisor
    Gold Member

    I would think that apples and oranges would have similar absorption properties! :biggrin:
  11. Aug 25, 2010 #10
    Well they certainly don't have the same reflection spectra, so actually, it suggests the same for the absorption spectra, doesn't it?

    (there's somewhat of a truth to the far-fetched analogy)
  12. Aug 25, 2010 #11


    User Avatar
    Science Advisor
    Gold Member

    I don't know, I figured that they have similar compositions and densities and that is all that matters for x-rays. Does a mirror that reflects optical light reflect x-rays as well?
  13. Aug 25, 2010 #12
    X-rays probably don't car much about fruit.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for Power laser Power Date
A Decreasing the power density of a laser Jun 3, 2017
I Decreasing Laser power Jun 2, 2017
I Understanding laser power terminology Mar 19, 2017
Laser beam propagation Feb 8, 2016
I How to estimate the power of a laser pointer Aug 14, 2015