1. Павел Алексеевич Черенков
And
Maxwell

2. Speed of light in a medium
I run slower in water than I do on a track. Similarly, matter impedes the movement of light, and light does not necessarily travel at c(=3 X 10^8 m/s) inside a medium, like glass, water, air, etc. ‘c’ is the speed of light in a vacuum.
frequency is constant
n=c/v, called the index of refraction. n>1 always.

4. Cerenkov radiation is to light as a sonic boom is to sound
This is a picture from inside a nuclear reactor (which is on). Nuclear fission produces high energy charged particles which move faster than light IN WATER (nothing goes faster than light in a vacuum).

5. Invisibility cloak
…ok, so n>1…but if I have a composite material, a metamaterial, which is made of normal physics-obeying matter then maybe I could build a negative refractive index (U.Tokyo, berkeley, UCL london, duke,etc)