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Why is a photon described as a packet of energy?

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A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

A photon is described as a packet of energy because, at the submicroscopic level, electromagnetic radiation is not emitted or absorbed as a continuous stream, but rather in discrete, localized units known as quanta.12

This concept emerged from the study of the photoelectric effect, where Albert Einstein proposed in 1905 that light consists of these discrete energy packets. This built upon the earlier work of Max Planck, who discovered that energy is exchanged in units proportional to the radiation's frequency, defined by the formula $E = h ν$ , where $h$ is Planck’s constant and $ν$ is the frequency.1345

While light exhibits wave-particle duality—behaving as a continuous wave in some contexts—it acts as a particle-like "packet" when interacting with matter. These photons are considered bosons and serve as the fundamental carriers of the electromagnetic field.321