Electromagnetic radiation: Why lower energy radiation( like radio waves) penetrates walls and higher energy radiation(like visible light) doesn't? How can microwaves pass through opaque objects in spite of having lesser energy than visible light(which doesn't pass through opaque surfaces)??
What makes an object "opaque" in the first place?
 A: What makes an object opaque, transparent, or absorbent of electromagnetic energy is the absorption mechanisms involved between matter and the photon.
Low frequency radio waves pass right through most objects. There is little or no interaction between the low energy photons and matter. In the case of microwave radiation the photon energy level corresponds to molecular rotation and torsion. So microwave energy can cause rotation of polar molecules such as water, which is the mechanism by microwave ovens cook.  
Infrared photon energy is in the range corresponding to molecular vibration. They are absorbed more strongly than microwaves but less strongly than visible light. Infrared radiation is the primary means for cooking in a conventional oven.
Visible light photons are absorbed strongly and their photon energy levels enable the elevation of electrons to higher energy levels, but not strong enough for the removal of electrons (ionization).
In the lower range of ultraviolet most of the uv from the sun is absorbed by a thin outer layer of your skin. Higher frequencies of uv are ionizing radiation that can produce harmful biological effects from sunburn to skin cancer. 
Finally, as you move up into the x-ray range the ionizing energy levels are sufficient to completely remove electrons from atoms
For a more detailed discussion of the interaction of radiation with matter, see the following link:
http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html
Hope this helps.
