Hi , well we all know that the speed of light in vacuum is measured at c and it slows down when it enters other mediums. Now we always say speed of light is c in vacuum but hence I believe the speed is c there because there is nothing in the way that could slow light down like there is in other mediums like water , air , different types of gasses etc. But we also know that there are different types of vacuums out there , some mediums are closer to a true vacuum and some are not , like there is a partial little vacuum in the intake manifold of a internal combustion engine but we wouldn't say that light would travel at c in there would we? So the question then is , how "empty" the vacuum needs to be for light to travel at c in it ? For example would the vacuum which is commonly used in a CRT tube is enough ?