Electromagnetic radiation is not a smooth stream of energy. It comes in indivisible quanta called photons, each carrying an energy fixed by its frequency and a momentum fixed by its wavelength.
A photon is a quantum of electromagnetic energy. Its energy is E = hf = hc/λ, often measured in electronvolts (1 eV = 1.60 × 10⁻¹⁹ J). Although a photon has no mass, it carries momentum p = E/c = h/λ. Higher frequency means more energy and more momentum.
Slide through the spectrum and watch the energy, frequency and momentum move together. Notice how a blue photon always beats a red one on every count, and how a small wavelength pushes you into the ultraviolet.
Everything in this lesson comes from these.
A photon's energy is set by its frequency, not by the brightness of the light. Making a beam brighter sends more photons per second, each still carrying the same hf. Keep wavelengths in metres when using E = hc/λ in joules, and convert to eV only at the end by dividing by 1.60 × 10⁻¹⁹.
Four quick checks on photon energy and momentum. Each correct answer earns XP and lights this skill on your star map.
The energy of a photon is directly proportional to its:
One electronvolt is equal to:
A photon of wavelength λ has momentum:
If the wavelength of a photon is doubled, its energy is:
When a calculation gives energy in joules, divide by 1.60 × 10⁻¹⁹ to convert to electronvolts; multiply to go back. Mixing the two is the most common slip in this topic.
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