The Einstein equation you are referring to is likely E=mc², where E represents energy, m represents mass, and c represents the speed of light in a vacuum. This equation is part of the theory of special relativity.
In the context of a photon, which is a massless particle, we cannot directly apply the equation E=mc² because the mass (m) of a photon is zero. However, we can still apply the principles of special relativity to photons by modifying the equation.
For a photon, we can use the equation:
E = hf
where E is the energy of the photon, h is Planck's constant, and f is the frequency of the photon. This equation is known as the photon energy equation, where the energy of a photon is directly proportional to its frequency.
Additionally, the momentum (p) of a photon can be calculated using the equation:
p = E/c
where p represents momentum and c is the speed of light in a vacuum. Since the energy of a photon is directly related to its frequency, we can also express the momentum of a photon using its frequency:
p = hf/c
These equations allow us to describe the energy and momentum of a photon without relying on mass. Keep in mind that while photons are massless, they still possess energy, momentum, and other properties that make them significant in the field of physics.