Photon Energy and the Electronvolt
Light delivers energy in packets of size hf, so the energy of a photon is fixed by its frequency alone. The electronvolt is the convenient unit at that scale: the energy one electron gains crossing one volt.
What a learner can do afterwards
- Calculates photon energy from a frequency or a wavelength using E = hf together with c = f lambda
- Converts between joules and electronvolts in both directions
- Estimates how many photons a second a stated laser power delivers
1 · Read
Light arrives in packets, each carrying E equals h f. Double the frequency and every photon doubles its punch. Frequency sets the punch of each packet, while brightness sets how many arrive. Ultraviolet triggers chemistry that dim red never will. Speed ties the two together: c equals f times lambda. One nanometre is 1 times ten to the minus 9 metres.
The electronvolt fits this scale: it is the energy one electron gains crossing a one-volt gap. Energy is charge times voltage, so one eV comes out to 1.6 times ten to the minus 19 joules. A 2.5 eV photon carries 4.0 times ten to the minus 19 joules. To convert, multiply or divide by 1.6 times ten to the minus 19, and mind the direction.
Count packets to link power and brightness. A 0.6 watt beam of photons carrying 3.0 times ten to the minus 19 joules each delivers 2.0 times ten to the 18 photons per second. Divide power by energy per photon and the rate drops out.
Fix each photon by frequency, price it in electronvolts, and count packets for brightness.
2 · Watch
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Where it sits
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.