The speed of light (when it is travelling in a vacuum) is 299792 kilometres per second. It is often approximated to 3*108 meters per second. Converting the unit of distance into miles, the speed of light in a vacuum can be written as 186282 miles per second. This speed is sufficient to travel the Earth’s circumference more than 7 times in a single second. The distance travelled by light in a year is called a light-year.
It is important to note that light travels in waves (in a fashion similar to the way sound travels). Any contact with matter can decrease its speed. However, its speed in the Earth’s atmosphere is almost similar to its speed in a vacuum.