# Why isn't the penetrating depth of electromagnetic radiation consistent with its frequency?

If I trust a quick Google search:

Microwaves at 2.45 GHz can penetrate 17 mm through muscle tissue (source).

Visible light only 1-5 mm and ultraviolet something like 0.2 mm (source).

But now X-rays can penetrate the whole body (because then X-ray and CT scans wouldn't be a thing).

And then the gamma rays come, which are famous for needing a 40 cm lead block to block most of it (source).

Why is there a "minimum point" of penetrating depth at UV? Why is it getting greater when increasing or decreasing the frequency?

• You only need classical EM to explain this Commented Sep 3, 2023 at 15:26
• Classical E&M does not predict absorption vs wavelength. Commented Sep 3, 2023 at 16:04
• The question is about the penetrating depth. Commented Sep 3, 2023 at 16:45
• Why indeed would an organism that lives in the sun's light evolve to block its most intense ionizing radiation? Commented Sep 4, 2023 at 3:32

Why the penetrating depth of an electromagnetic radiation isn't consistent with its frequency?

Because the energy levels corresponding to absorption and penetration, and the effects of that absorption at the atomic and molecular levels, are quantized.

If there are no available quantized energy levels of the particular material to match the quantum energy of the incident radiation, then the material will be transparent to that radiation. This makes the degree of absorption and penetration not necessarily proportional to the frequency.

For example, considering the human body, low frequency radio waves are essentially transparent. Absorption increases strongly between microwave frequencies and visible light. Ultraviolet is essentially all absorbed by the outer thin skin. Moving up to the x-ray region the body is transparent again with only a small fraction being absorbed, though the absorption involves violent ionizing (potentially cancer producing) radiation.

For more details on the interactions with the human body and their effects, see: http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html

Hope this helps.

There are many processes involved in penetration of matter by electromagnetic radiation. At 2.4 GHz, the principal mechanism limiting the penetration depth is often ohmic dissipation of induced ionic currents. A microwave oven doesn't heat pure water very effectively, but add a bit of salt to the water, and the effect becomes stronger.

At shorter wavelengths, molecular vibrations and electronic transitions play a role. The bands where these effects occur may be wide (depending on the material), but they generally decline out of band.

When you get to UV, photoionization is the major mechanism. For any particular ionization pathway, the pattern is that the attenuation jumps up when the wavelength becomes short enough that the photon energy is sufficient to remove an electron from an atom at its particular energy. At higher energy/shorter wavelength, the attenuation due to that pathway declines. However, as you reduce wavelength through the ultraviolet to the soft x-ray, more electrons become available for ionization. More pathways->more opacity. Once you get to the soft x-rays, though, there are few additional pathways left, while the opacity due to each pathway is declining. So, opacity of materials is usually greatest in the UV.

• Re "A microwave oven doesn't heat pure water very effectively": Isn't it set at some resonant frequency (or not)? Commented Sep 4, 2023 at 14:20
• @PeterMortensen After more reading, it appears that both the resonant claim and the salt claim are urban legends. Non-resonant dielectric absorption appears to be the mechanism. Commented Sep 4, 2023 at 15:46
• " A microwave oven doesn't heat pure water very effectively, but add a bit of salt to the water, and the effect becomes stronger." John, in all the years I've worked on microwave oven with respect to safety, i never heard that. Can you elaborate or give a reference? Thanks. Commented Sep 4, 2023 at 23:28

A possible ELI5 explanation as I look at this:

For long wavelength radiation (big photons) you are thin. They pass through a wall barely noticing it. You are a speck.

For short wavelength radiation (tiny photons) you are mostly made of holes. Like a radio dish doesn't block a lot of visible light, your body doesn't block a lot of X-rays. They are so small they fly between the atoms.

No wonder our eyes evolved to detect the light they can actually interact with!

• I apologize for this, it may be very wrong. I just felt like the existing answers didn't give a good intuition for why this is happening. While my answer gives an intuition, it may be totally wrong. Commented Sep 4, 2023 at 21:05