Objects being transparent or translucent depends on various factors. An object could be transparent to short radio waves but not to the visible light. Let's consider the discussion with respect to visible light. To see the difference between these terms lets look at the process of how light or photons interact with matter.
Electrons in objects have different energy levels and as per Plank's theory energy exists in chunks called quanta. This is how energy levels are created where the electrons could be present.The energy levels available for electrons are discrete and are different for different objects.
When light or photons are incident on an object the photon might get absorbed in the body or pass through it. Light components have different wavelengths and the energy depends upon it.
E = hv where v is the wavelength and h is Plank's constant.
If the energy levels available for the electrons have a gap equal to E i.e if the photon has energy exactly equal to that required to bridge the energy level gap, the photon gets absorbed.
However If higher energy levels do not exist or are too far apart for the energy of the radiation to bridge the gap the object would be transparent to the radiation.
This is the case with glass where visible light does not have enough energy to get absorbed into it. However glass may not be transparent to higher energy radiation. Adding a small impurity may reduce the gap between energy levels and allow some frequencies to get absorbed. This also gives the light coming out of glass a distinct color depending upon which frequencies did not get absorbed. Thus adding significant amount of impurity hinders light components and makes the object translucent.
