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# Random physics question - help! watch

1. 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!
2. Not 100% sure, but high energy photons have high frequency and therefore short wavelengths. With shorter wavelengths there will be less attenuation me thinks. The radiation is more concentrated and therefore less reflection.
3. Its to do with the fact that they have a high kinetic energy thus leading to a quicker velocity as Ek=1/2mv^2. Quicker velocity allows the high energy particle to penetrate the materials. Hope that answers your question!!
4. (Original post by supercacti)
Its to do with the fact that they have a high kinetic energy thus leading to a quicker velocity as Ek=1/2mv^2.
It's true more energy means faster speeds for particles with mass, but photons don't have rest mass, they always move at the same speed.

Also, for high speed particles E = 0.5mv^2 isn't really applicable. If it was you'd find your TV or computer screen could accelerate electrons to faster than light. You need a different equation when dealing with high speed anything.
5. Also akin to the reason that themo (slow) neutrons are better at initiating fission. Neutrons with high momentum has a short De Broglie wave length. Hence it is as if the neutron is spread out less when traveling, and hence less likely to interact with matter, as it has to pass within the range of the strong nuclear force for it to initiate fission.
A photon with high momentum also has a short wavelength, hence spread out less. Since photons are a boson, they can not interact with photons, and hence need to pass very close to any matter it interacts with.
6. Its also to do with the energy levels of the system. A system will absorb photons whose energy corresponds to the gap between energy levels of the system. Visible light has about the right energy; Xrays (and radio for example) dont so they go through.

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Updated: February 2, 2006
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