1248. I know that the medium of electromagnetic waves is a photon. What is a photon? What is it made of? — ON, Istanbul, Turkey
First, an electromagnetic wave consists of an electric and a magnetic field. These two fields create one another as they change with time and they travel together through empty space. An electromagnetic wave of this sort carries energy with it because electric and magnetic fields both contain energy. That much was well understood by the end of the 19th century, but something new was discovered at the beginning of the 20th century: an electromagnetic wave cannot carry an arbitrary amount of energy. Instead, it can carry one or more units of energy, units that are commonly called "quanta." An electromagnetic wave that carries only one quanta of energy is called a "photon."
The amount of energy that a photon carries depends on the frequency of that photon—the higher the frequency, the more energy. Photons of visible light carry enough energy to induce various changes in atoms and molecules, which is why they provide our eyes with such useful information about the objects around us—we see how this visible light is interacting with the world around us.