Here’s a delicious contradiction: sometimes, light is a wave and sometimes it behaves like a stream of particles.

We think of light as the visible spectrum that gives us color and makes darkness go away. Scientists see a much broader spectrum of electromagnetic radiation spanning from very long radio waves to very short gamma waves.  Light transports energy at a constant speed of 299,792,458 meters per second (186,282 miles per second). (That's fast. It is also, far as we can tell, the universal speed limit.)

Science describes light as an Electromagnetic Spectrum varying in wavelength (the distance between wave peaks) and frequency. 

How was the speed of light measured?

In 1676, Ole Rømer timed eclipses of Jupiter’s moon Io and noticed delays when Earth moved away from Jupiter.

In 1728, James Bradley used stellar aberration—the apparent shift in star positions caused by Earth's orbital motion—to refine the speed calculation.

In 1849 Hippolyte Fizeau shone a bright light through a rapidly spinning toothed wheel to a distant mirror eight kilometers away and timed the round trip.

Between 1826 and 1931, Albert Michelson used precisely rotating mirrors between mountain stations to improve measurement accuracy.

Let's dig down into this
particle wave business

Light behaves as a particle when it interacts with matter through the absorption or emission of energy. These discrete packets of energy, called photons, manifest during specific quantum events and physical phenomena. For example, in a phenomenon called 'quantum detection', when light lands on a digital camera sensor like a CCD or CMOS chip, it is detected as individual “hits” or dots rather than as a continuous wave.

Light behaves like a wave when it travels through space and interacts with obstacles. Examples: When light passes from air into water or glass, it slows down and changes direction. This is called refraction. In experiments passing light through tiny gaps, it bends and spreads out. This is called diffraction. When light waves cross paths, they merge and cancel each other out. This creates bright and dark bands called interference.