Why does light move in waves?
Light does not "move" in waves. Light IS an electromagnetic wave, or electromagnetic radiation, which is created by the changing electric and magnetic fields. These, in turn, can be created by accelerating electric charges, alternating electric currents, and changes in the structure of atoms or nuclei of atoms.
The reason why electromagnetic radiation propagates, or moves (for example, a wire with alternating current in one place creates electric and magnetic fields that travel through the surrounding space and induce a current in a wire far away), is described by Maxwell's equations. These are the four laws of electric and magnetic fields:
- Gauss' Law: Stationary electric charges create an electric field
- Magnetic charges, or monopoles, do not exist.
- Faraday's Law: Changing magnetic flux creates an electric field.
- Ampere's Law: Magnetic field is created by moving charges, or currents. Maxwell made an addition to that law that states that magnetic field can also be created by changing electric flux. He suggested this from theoretical considerations, but this fact was later confirmed experimentally. Because this addition resulted in the realization that light is an electromagnetic wave, the four laws together are named after Maxwell.
The solution of these for laws, of equations, is an electromagnetic wave that propagates with a certain speed, which depends on the electromagnetic properties of the medium. In the vacuum, this speed happens to be `3*10^8` m/s, which was already known, at the time of Maxwell, to be the speed at which the light travels. This led to the understanding that light is a propagation of electromagnetic field.
The visible light is the electromagnetic wave with the wavelength somewhere between about 400 and 700 nanometers (a nanometer is a one-billionth of a meter.) This is a very narrow part of an entire electromagnetic spectrum, which ranges from the waves with the wavelength of several kilometers, such as radio waves, to the gamma rays, which have the wavelength of a few Fermi ( `10^(-15)` of a meter).
Please read more about the phenomena that demonstrate the wave-like nature of light on the referenced website.