Photon energy
Light comes in tiny packets called photons. A photon's energy is E = hc/λ, where h is Planck's constant, c is the speed of light and λ is the wavelength. Shorter wavelength means more energy.
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Analogy
Like coins: blue light is a heavy coin, red light is a light one. You can't pay with half a coin.
Example
λ = 500 nm = 5.00×10⁻⁷ mE = (6.626×10⁻³⁴ × 3.00×10⁸) / 5.00×10⁻⁷E = 3.98×10⁻¹⁹ J
In practice
UV light can damage skin because each UV photon carries enough energy to break chemical bonds, while a red photon cannot.
Practice
Flashcards
- What equation links a photon's energy to its wavelength?E = hc/λ. Shorter wavelength means higher energy.
Questions people ask
Does brighter light mean more energy per photon?
No. Brightness depends on how many photons arrive each second. Energy per photon depends only on the wavelength. A dim blue lamp gives fewer photons than a bright red one, but each blue photon still carries more energy than each red photon.
Why do we use joules per photon and kilojoules per mole?
One photon's energy is tiny, around 10⁻¹⁹ J, so it is awkward to use. Chemists multiply by Avogadro's number, 6.022×10²³, to get the energy of one mole of photons. That gives kilojoules per mole, which can be compared directly with bond energies and reaction enthalpies.