Radiowaves, microwaves, and visible light are all part of the same big spectrum.

 

If the only difference between these spectrums is wave-frequency, then how is it that some waves are able to pass through objects (like radiowaves), and light waves are blocked? How does the quantitive change of frequency cause a qualitative change of being able to pass through matter?

Different materials have different electron structures. If there are no transitions that would absorb the EM wave, or they are weak transitions, then the wave can pass through. (assuming the photons are not abe to ionize the molecules)

What do you mean by "transitions"?

I believe he means transitions through electron energy states. Assuming I'm right, this is also called up-conversion and excitation. I'm sure there's more names for it, too. Basically, it's the frequency that an electron needs to change its energy level.

 

Wow, I hope I got all that right... Something in my brain doesn't function right when I'm this tired

How would a person go about predicting transparency of different things to certain wavelengths? Or is it too complex, considering how many possible compounds there are out there.

You have to calculate resonant frequencies of the particular molecules. Absorption is high near a resonance. I guess this is done by molecular chemists, which I am not.

'']How would a person go about predicting transparency of different things to certain wavelengths? Or is it too complex, considering how many possible compounds there are out there.

 

I'm not sure what the state of the art is, but I don't think the QM models are up to the task. Once you form larger molecules the state structure gets very messy. I think it's done the other way around - take a substance and measure its spectrum - but I can't say for sure.