# Density and wave absorbtion

#### Markysims

Hi,
Is there an equation or a formula or ratio or something similar that relates the density of an object to how much it absorbs energy from the wave moving through it?

Thanks

MarkySims

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#### Claude Bile

The closest law I can think of that does this would be Beer's Law.

$$A=A_0e^{-\alpha.z}$$

The quantity $\alpha$ depends on the absorption cross section, which is a function of density (I can't think of the exact relation off the top of my head).

Note that such a law could only be used to compare two samples of the same substance as absorption cross section varies from substance to substance.

Regards,
Claude.

Edit - This applies to electromagnetic waves, but I cannot be 100% certain in applied to other types of waves, maybe someone else can clarify this.

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#### mathman

For very high energy radiation (gamma rays) it is more complicated. In addition to absorption, the gamma rays undergo scattering, which changes the ray direction and also results in a loss of energy. This subject has been heavily investigated and I presume there are tables of the energy attenuation for various substances.

#### amt

I think Plank's constant plays a role here. Because, how much energy is absorbed will depend on the frequency of the wave passing through.

#### Claude Bile

Electronic and rovibrational resonances of the substance in question will dominate the absorption spectra, variables like density will probably only be significant when comparing two samples of the same substance.

Regards,
Claude.

"Density and wave absorbtion"

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