What is a photon? Show that it has zero rest mass or photons can not exist at rest. Explain.

Explanation

### Step-by-Step Solution

**Step 1: Definition of a Photon**
A photon is defined as a quantum of electromagnetic radiation, which can be thought of as a packet of energy. It is the fundamental particle of light and carries energy proportional to its frequency.

**Step 2: Energy of a Photon**
The energy (E) of a photon can be expressed using the formula:
\[ E = h \nu \]
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