Doesn't a "wave" require a "medium" through which to propogate?

 Physically speaking, sound is transported by the collisions of molecules. The easier it is for molecules/atoms, to collide (higher density of the medium) the faster sound travels. Above 100 km altitude, the average distance between air molecules is so high, that these molecules don't collide on a regular base and spend most of the time flying in their own orbits, just like very small satellites. No collisions, no transport of sound. 

Just as the velocity of sound depends on the density of the medium through wch it is being propogsted, the velocity of light also varies depending on the medium through wch it is propogating.

Thus the speed of light is not constant ,but changes according to the density of the medium (and temperature) it is traveling through.
Light has a constant speed of 3 x 10^5 km/second = 299,792,458 meters/second = (186,282 miles/second) =(9.5 trillion kilometers /year) through the medium of a vacuum or empty space(according to quantum physics even a perfect vacuum is not empty,but teeming with virtual particles that are popping in and out of existance over very short time frames.This suggests that so-called "empty space" is actually permeated with unobservable energy and invisible forces that are inaccessible to our senses.The energy density of empty space is immense and might account for the missing "dark energy" that is unobservable.If we could tap into this hidden store of energy we would have an unlimited source of fuel/energy.However no one has yet come up with an experimental method to weigh a cubic meter or more of "empty" space in order to measure the weight/mass-equivalence of the virtual particles contained therein).

How can massless energy(photon) materialize into a particle with mass and inertia?

If the speed of light diminishes with increasing density of the medium through wch it travels, at what density of matter would the velocity of light become zero?).
Perhaps the reason light cannot emerge from a Black Hole has something to do with the very high density of it's content/composition?
Light slows to about 225,000 km (140,000 miles) per second when it goes through water( speed of light in ice is 2.29x10^8 m/s) ,and about 206,000 km (128,000 miles) per second when it goes through quartz crystal and 124,000 miles per second through glass. A diamond, , will slow light to 125,000 km (78,000 miles) per second--this is less than half the speed through vacuum!).
Composite white light separates into its constituent frequencies upon passage through a transparent prism because each frequency is slowed down to a different degree according to its energy content.

But photons don't really slow down.rather each hits an atom,gets absorbed and another gets emitted. This process takes different amounts of time depending on the medium and the energy content (frequency) of the photon,hence the time delay.The photon that strikes the window is not the one that emerges on the other side.Is this a complete explanation of the mechanism by wch light slows down when it enters a denser medium?

Light moving through a Bose-Einstein Condensate ( a state of very condensed matter found at near absolute zero)travels at very slow speed. What determines this difference between how much light is slowed?Lene Hau was able to slow light to 17 metres per second by passing it through a transparent superfluid --Bose-Einstein Condensate--a state of matter that only exists at near absolute zero temperatures.Dr. Hau and her associates at Harvard University have successfully transformed light into matter and back into light (was able to momentarily stop/capture a beam of light)using Bose–Einstein condensates. Details of the experiment are discussed in the February 8, 2007 publication of the journal Nature (see entry in this journal titled "Light and Matter United")
Since most of space is within a few degrees of absolute zero (the temperature at wch Bose-Einstein Condensates occur)it is not outside the realm of possibility that light has traversed such a state of matter in reaching us .Such an occurrence would necessarily have slowed it's speed unpredictably and makes "distance to origin" calculations of questionable acracy!This means that light passing through a cloud of dust,gas , plasma,or a Bose-Einstein Condensate (i.e.,non-empty space) would have it's path refracted and/or slowed thereby giving an observor a false position for the emitting object.

Can we ever accurately know the exact (versus apparent ) position of light-emitting objects in the night sky?