Electromagnetic wave properties of light vs radio waves Both light and radio waves are electromagnetic waves. That means they have almost similar properties.
Both are EM waves, $E$ and $H$ fields are there in both waves... but
My question is: "radio waves can pass through walls, but why can't a light pass through walls or opaque medium???"
 A: It has to do with the frequency response of the materials in the wall. Different molecules absorb different frequencies (or wavelengths) producing an absorption curve called the materials spectral response. Lots of materials are very absorptive in the frequencies typical in visible light but start to open up (get clearer) in longer wavelengths. Generally the energy of the visible photons are higher than the RF photons so they can react with more stuff and get absorbed. The lower energy radio photons can't react so they pass thru. All bets are off with metal walls for both, but that's a different effect, reflection not absorption.  
A: One of the defining characteristics of an electromagnetic wave is its wavelength (which is related to its frequency). Radio waves have wavelengths ranging from 1 millimetre to 100 kilometres, while light has wavelength on the order of hundreds of nanometres.
Interaction between electromagnetic waves and objects can be roughly predicted with the relationship between wavelength and the object's size; if the wavelength is greater than the object's largest dimension, then the object is invisible to the wave. If the wavelength is smaller than the object's largest dimension, then the object is "visible" to the wave, and there will be interaction between the two (usually in the form of reflection and refraction). If the wavelength and the object's largest dimension are similar, then a complicated process called scattering or diffraction occurs.
