That depends.
It’s often said that the speed of light is constant, but it isn’t. Only in a vacuum does light reach its maximum speed of nearly 300,000 km per second (186,282 miles per second). In any other medium, the speed of light varies considerably, always being slower than the figure everyone knows. Through diamonds, for example, it goes less than half as fast: about 130,000 km per second, or 80,000 miles per second.
Until recently, the slowest recorded speed of light (through sodium at -272°C) was just over 60 kph (38 mph): slower than a bicycle.
In 2000, the same team (at Harvard University) managed to bring light to a complete standstill by shining it into a bee (Bose-Einstein condensate) of the element rubidium.
Rubidium was discovered by Robert Bunsen (1811-99) who didn’t invent the Bunsen burner which is named after him.
Astoundingly, light is invisible.
You can’t see the light itself, you can only see what it bumps into. A beam of light in a vacuum, shining at right angles to the observer, cannot be seen.
Although this is very odd, it’s quite logical. If light itself was visible, it would form a kind of fog between your eyes and everything in front of you.
Darkness is equally strange. It’s not there but you can’t see through it.
(From “The Book of General Ignorance”)
Filed under: science
