Re: Why does reflection of light/sound depend of wavelength?

Your comment about whether the reflection is unique to electromagnetic 
radiation or does it apply to other waves is a good one:  "Waves are waves" 
and the phenomena of reflection and refraction and scattering are common to 
all waves whether they are EM or sound.

Now, as to why the reflection depends on wavelength.  The relevant size is 
the width of the object as seen by the wave, and where the "edges" are.  If 
the edges are "far away" (i.e., more than 5-10 wavelengths) then for all 
practical purposes they will have no effect on what happens at the point 
where the wave hits the surface, and therefore we make the approximation 
that the surface is infinite in extent and model the interaction as a 
"reflection", and is independent of frequency.  This is like light 
reflecting off the surface of a lake.

If the edges are very close together (i.e., the object is much smaller than 
a wavelength) then we use a model of "scattering", in which we calculate 
the strength of the wave scattered in all directions based on matching 
boundary conditions at the surface of the object.  This turns out to be 
frequency dependent, usually with high frequencies being more efficiently 
scattered than are low frequencies, leading to the "Blue Sky Law" of 
optics.  This regime is like light scattering from gas molecules, each one 
of which is smaller than a wavelength of light.

The in between range, from 10 wavelengths down to .1 wavelength, is the 
interesting regime, in which we have to make all sorts of complicated 
calculations to model the behavior of waves.  This can lead to constructive 
and destructive inteference, standing waves, diffraction of waves around 
the object, and other interesting phenomena.

There is no size of object that will have no effect on the wave.  Waves do 
not pass through a small object, but rather the magnitude of the effect and 
the way it is calculated is different.