---
title: "Photon Energy and the Electronvolt"
description: "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 on"
canonical: https://lightmysky.com/learn/science/photon-energy-and-the-electronvolt-mt_UfmDhGRaYT
source: https://lightmysky.com/learn/science/photon-energy-and-the-electronvolt-mt_UfmDhGRaYT.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Waves, Light & Sound · Ages 17 to 18
Page: https://lightmysky.com/learn/science/photon-energy-and-the-electronvolt-mt_UfmDhGRaYT

## Ready when they can

- 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

## Lesson: Packets of light, priced per photon

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.

**Recap.** Fix each photon by frequency, price it in electronvolts, and count packets for brightness.

## Practice

13 questions on this page, each with its working shown.

## Needs first

- [Electric Potential and the Uniform Field Between Plates](https://lightmysky.com/learn/science/electric-potential-and-the-uniform-field-between-plates-mt_LKTfeYxNde)
- [The Electromagnetic Spectrum: Uses and Dangers](https://lightmysky.com/learn/science/the-electromagnetic-spectrum-uses-and-dangers-mt_P_NXNSo1zQ)
- [Single-Slit Diffraction and the Limit of Resolution](https://lightmysky.com/learn/science/single-slit-diffraction-and-the-limit-of-resolution-mt_qiG0fGadBU)

## Opens up

- [Particles, Antiparticles and Annihilation](https://lightmysky.com/learn/science/particles-antiparticles-and-annihilation-mt_-Ud20HRNJz)
- [The Photoelectric Effect and the Work Function](https://lightmysky.com/learn/science/the-photoelectric-effect-and-the-work-function-mt_ADQK7WB2JN)
- [Blackbody Radiation and the Quantum of Energy](https://lightmysky.com/learn/science/blackbody-radiation-and-the-quantum-of-energy-mt_B2Bk2IAOa9)
- [Mass, Energy and the Unified Atomic Mass Unit](https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X)