Strange wavelength paradox?

[tuesdaysgreen]

It's a fact that longer wavelength/lower frequency light (e.g. red) penetrates dense gas clouds better than high frequency/short wavelength light (e.g. blue). It's also a fact that you require a less dense surface to capture and focus longer wavelength light (e.g. radio) which is why radio telescopes have holes in them and it doesn't affect the instrument. You could do radio astronomy with a dish made essentially of chicken wire, but you could not do optical astronomy that way.

My question: Why should a denser gas cloud be porous to long wavelengths and a less dense detector be opaque to them?

[cjameshuff]

My question: Why should a denser gas cloud be porous to long wavelengths and a less dense detector be opaque to them?

The one is basically the inverse of the other. A radio telescope's mesh is a surface with small holes in it, a cloud of dust or gas is a bunch of empty space with small objects in it. If a hole is too small for radiation of a given wavelength to pass through, a particle of the same size will be too small to block it. (This is a simplification, of course...radiation will interact with gases even though the molecules are far smaller than a wavelength, etc.)

[ngc3314]

Furthermore, the integrated density (atoms/cm^2 or g/cm^2) of a typical interstellar cloud is much less than even the average of a mesh radio reflector. The senses of the comparison for the two cases can be a bit misleading.

[antoniseb]

The one is basically the inverse of the other. A radio telescope's mesh is a surface with small holes in it, a cloud of dust or gas is a bunch of empty space with small objects in it. ...

Adding to the idea here, the mesh is electrically conductive, and so can operate as one big object.

Interestingly short wavelength photons (gamma rays) can also penetrate the dust clouds better than optical photons can. They are too small to be diffracted by the dust.

[mugaliens]

In the wee hours I'd posted a detailed response!

Now, it's gone.