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Weird physics question watch

    • Thread Starter

    hi, i've just finished my A2 physics coursework and realised that i can't explain something which seems really common-sense...

    why do higher-frequency (higher-energy) photons pass through materials more easily than lower-frequency photons? so why are x-ray photons not so easily absorbed by electrons in patients' bodies than visible-light photons are?

    am i just being really thick? i seriously can't explain it...

    thanks a lot!

    Debroglies formula is p = \frac{h}{\lambda} = \frac{hf}{c}

    Therefore, the higher the frequency, the more momentum the photon has. For Xrays and gamma rays, they have millions of times more momentum than visible light.

    Another consideration is that the typical bond lengths in materials are not of a compearable length to the wavelength of xrays or gamma rays. This is the reason why glass is transparent to visible light, but not to infrared, because the bond lengths in glass are more about the same length as IR, not visible light.


    WTF happens to the latex?!

    (Original post by mrmatty)
    why do higher-frequency (higher-energy) photons pass through materials more easily than lower-frequency photons?
    They don't, necessarily. The lowest frequency possible, radio waves, will pass quite happily through walls. Microwaves will go through plastic and wood quite happily, but are stopped by a thin sheet of metal. Visible light is stopped by the vast majority of solids, but will pass straight through the vast majority of liquids. Ultraviolet can either be stopped or not stopped by glass, depending on the wavelength, but is blocked by a layer of suncream a fraction of a millimeter thick that is completely transparant to visible light. Etc, etc.

    In other words, there's no definite rule. If a material has some property that causes light of a certain wavelength to be absorbed (say, Alphanumeric's example where the bond length is comparable to a wavelength, or the energy level by E=hf is just right to jump an electron to a higher energy level), then it will be absorbed. It is true that gamma rays tend to pass through almost anything, but then, radio waves pass through quite a lot, too - and their momentum is incredibly tiny compared to gamma rays.

    In fact, I seem to remember reading somewhere that the reason visible light is in the region that it is is because, in evolutionary terms, it is advantageous to be able to see where things are, so the part of the spectrum that we evolved to see was a part that is opaque to most solid materials. If someone had eyes that could see only IR, for example, they could see and avoid warm sabre-toothed tigers but would keep walking into (colder) rocks and trees.
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