Electromagnetically Induced Transparency is a Nobel Prize-winning technique
of reducing the light absorption of a solid object by shining electromagnetic
waves (in this case - microwaves) on it, tuned to the resonance frequency of
the object. What this does is actually excite the electrons of the atoms inside
the object to certain energy levels. The change in structure in the atoms allows
another beam of light - a probe laser, for instance, to pass through the object
at a particular angle.
This technique has been used by physicists to observe the slowdown effects
of light waves through objects, and the refractive index (the measure of how
much the object slows and bends light).