What is a photon and what is it's function and where is present

In physics, aphotonis a bundle of electromagnetic energy. It is the basic unit that makes up alllight. Thephotonis sometimes referred to as a "quantum" of electromagnetic energy.Photonsare not thought to be made up of smaller particles. They are a basic unit of nature called an elementary particle.
Helium atoms have two electrons orbiting the nucleus. ... Aphotonis produced whenever an electron in a higher-than-normal orbit falls back to its normal orbit. During the fall from high energy to normal energy, the electron emits aphoton-- a packet of energy -- with very specific characteristics.