# Thomson Cross Section Constant

Use Thomson Cross Section in Calcul.io. See what this built-in constant represents, enter it in an editable calculation, and review related values and units.

## thomsonCrossSection

Thomson cross section

Try it yourself:

```calculio
thomsonCrossSection
```

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## Thomson cross section for low-energy scattering

**thomsonCrossSection** is the classical cross section for scattering electromagnetic radiation by a free electron at low photon energy.

## Use

It provides a reference area in plasma and astrophysical scattering estimates. It depends on the [electronMass](https://calcul.io/constants/electronMass/index.md), [elementaryCharge](https://calcul.io/constants/elementaryCharge/index.md), and electromagnetic constants.

## Caveats

It applies in the classical, low-energy limit. At high photon energies, quantum Compton scattering is required. Treat a cross section as area, not a geometric particle size; use [multiply](https://calcul.io/function/multiply/index.md) with number density and path length only in the appropriate model.

[All constants](https://calcul.io/constants/index.md)