What is a photon?

A photon is defined as a quantum of electromagnetic radiation, meaning it is the smallest discrete amount of energy associated with electromagnetic waves. Photons are fundamental particles that have no mass and travel at the speed of light in a vacuum. Each photon carries energy that is proportional to its frequency, according to the relationship established by Planck's equation, which states that energy equals Planck's constant multiplied by frequency.

This definition highlights the essential nature of photons as the carriers of electromagnetic force and their role in various physical phenomena, such as light and other forms of radiation, while also clearly distinguishing them from sound or subatomic particles. Other options, while covering aspects of physics, do not accurately describe what a photon is. For instance, sound is a mechanical wave that requires a medium, subatomic particles refer to particles like protons and neutrons that make up atoms, and waves in the electromagnetic spectrum refer to the broader behavior of the waves rather than individual quanta of energy.