What is a photon? Several selections may be correct.

Has an energy of equal to hν.

Is a unit of electromagnetic radiation.

Is a part of the electromagnetic spectrum.

Is usually called visible light.

None of these.

Has an energy of equal to hν.

Yes.<
Is a unit of electromagnetic radiation.
Initially I wrote no; however, I GUESS that if we say that electromagnetic radiation is composed of photons; perhaps that is the same as saying that a photon is a unit (two hotons, 10 photons, etc) but this is a strange way of saying it.

Is a part of the electromagnetic spectrum.
An individual photon fits somewhere on the electromagnetic spectrum.

Is usually called visible light.
No, although visible light is composed of photons.

The correct answer is:

- Has an energy equal to hν.

A photon is a fundamental particle of light and other forms of electromagnetic radiation. It is considered to be both a particle and a wave. According to the photoelectric effect equation discovered by Albert Einstein, the energy of a photon is directly proportional to its frequency, denoted by v or ν. The energy of a photon can be calculated using the equation E = hν, where E is the energy of the photon and h is Planck's constant.

To find the answer to this question, you can eliminate the incorrect options one by one:

- "Is a unit of electromagnetic radiation": This is not correct because a photon is not a unit but a particle.

- "Is a part of the electromagnetic spectrum": This option is partially correct as photons do indeed make up the electromagnetic spectrum, but it does not fully define what a photon is.

- "Is usually called visible light": This is incorrect, as a photon represents the smallest quantity of light, not just visible light. Photons can exist across the entire electromagnetic spectrum.

Therefore, the correct option is "Has an energy equal to hν."