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Q & A: radiation penetration depth versus frequency

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Most recent answer: 02/10/2013
why do photons with more frequency more probably enter a substance (for example gamma ray can penetrate human body but visible light can not)?
- Sara (age 20)

How light interacts with material objects depends largely on the atomic structure of the medium and the frequency of light. For example, gamma rays penetrate human bodies with ease, while visible light is scattered or absorbed, and x-rays penetrate our soft tissues but are absorbed by our bones. How far each frequency of radiation can penetrate a given tissue depends on the resonance frequencies of its atoms.

We can give a sort of classical picture that captures some of the physics. Near resonance, the electric fields in a light wave exert forces on the atoms at just the right rate to build large amplitudes of oscillation. This can be understood by analogy with pushing a child on a swing-set. To make the child swing high in the air, one must push at the exact right intervals— that is, when the swing is just beginning its descent. Pushing at any other times will affect the swing very little (or even slow it). After multiple pushes at the right frequency, the swing will have built large oscillation amplitudes, just like an atom in a light beam at resonant frequencies. Now, the friction in the swing's bearings will slow the oscillations, and the "pusher" will become tired if he maintains his efforts for long. Similarly, the light expends energy via dissipative forces among the atoms it excites; the larger the oscillations, the more the energy loss. At much higher (i.e. gamma) or lower (i.e. radio) frequencies compared to resonance, the light barely induces any atomic oscillations at all, like someone who pushes and pulls randomly on the swing. Thus, little energy is lost to dissipation, and the radiation can penetrate the medium with ease. David Schmid

The very low frequency radiation turns out to be  absorbed in a different way. The big electrical currents of the salt ions in your body also pull energy out of the wave and dump it as heat in your body, just as water is heated in some electrical steamers by a current passing through it.

There are some special frequencies that happen to pass rather well through the body. For example, you can see a bit of red light through your fingers if you hold them in front of a light in a dark room. (see   for a discussion) There's quite an interesting pattern of which infrared frequencies are easily transmitted through our skin and which aren't: . Mike W.

(published on 02/10/2013)

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