- Aug 12, 2009
The space is a "near vacuum" media, so the less matter it has the less visibility of the beam will be seen. In a perfect vacuum light travels at the constant speed c. When a photon interacts with matter it is absorbed by the atom (actually a bound electron), and sometimes it is emitted away some time AFTER. The energy of an absorbed photon turns into movement of the atom (burning), and when re-emitted maybe in a different angle and/or different wavelenght (fluorescency) and depending of the nature of the matter being entered sometimes the photon is reemited the same way it was absorbed, allowing the light to travel across the matter (transparency) or even be reflected. When the light enters a solid/liquid/gas this event occurs LOTS of times delaying "apparently" the speed of light. But this is only apparent, the speed of the photon along its way from atom to atom is always the constant speed of light c. The apparent delay comes from the interval the light is absorbed/reemited from atoms, that is actually a little amount of time, but not zero. So, depending of the kind of matter this delay could be more or less producing a slightly different "apparent" speed of light.