Is the speed of light constant? The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light *can* slow down slightly when it passes through an absorbing medium ...

A series of new animations by a NASA scientist show just how zippy - and also torturously slow - the speed of light can be. Light-speed is the fastest that any material object can travel through ...

Related: Why is the speed of light the way it is ... they would appear to be moving in slow motion.