When an artificial intelligence (AI) model was asked the question in the title, the answer was "Under all conditions." This response is understandable because, in the dominant paradigm, photons have no size, structure, or mass, and consequently, there is no photon model, but there is spin. When asked about spin, the AI model replied that spin is not the rotation of matter in space, but rather a representation of the Lorentz group. Planck's formula is a postulate, not the result of a physical consequence. The answer to the question in the title can be found within the framework of a new paradigm built on Tesla's insight. In this new paradigm, photons have size, structure, non-zero mass, and spin; thus, compared with the dominant paradigm, the new paradigm provides a closer match to classical physics.Here, based on this photon model, we derive a generalized formula for the energy and frequency of a photon, in which Planck’s formula is a special case.