and they are on opposite ends of the electromagnetic spectrum, then why can't light travel through walls which is right in the middle of the spectrum?  

This question has already been asked [here](http://www.physlink.com/Education/AskExperts/ae175.cfm?CFID=38736108&CFTOKEN=686d5d990db4e8a0-A73FF6D5-15C5-EE01-B960081244D63FF9). However, I am not entirely satisfied by the answer. I am not looking for fanciful analogies and metaphors. I just want to know factual technical reasons why this happens.
If given a radio source and a gamma source of equal intensity, then will the radio source have more penetrability than the gamma source per formula for [penetration depth](http://en.wikipedia.org/wiki/Penetration_depth)? If not, why not?