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Can light bounce off an electron like a billiard ball? Comptonne’s photon strikes a resting electron, which recoils, and the photon leaves at an angle with a longer wavelength: Δλ = (h/mc)(1 − cos θ). It loses energy, not speed. Arthur Compton’s 1923 X-ray measurements showed that photons carry momentum. Pixel-art unisex tee; equation, symbols and sources in the physics panel below.

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Size guideS–5XL · inches

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Size & fitS–5XL

Unisex heavy cotton (Gildan 5000), classic fit.

Unisex tee size chart, inches
SizeWidthLengthSleeve
S182815.1
M202916.5
L223018
XL243119.5
2XL263221
3XL283322.4
4XL303423.7
5XL323525

Measurements in inches, ±1.5 in tolerance.

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Free US shipping · $10 flat shipping outside the US. Printed to order for you.

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Returns & replacements21 days

Every item is printed to order, so we can’t accept returns or exchanges for change of mind, or if you ordered the wrong size or colour.

Print defect, misprint, damage or the wrong item? We’ll send a free replacement, or a refund if you prefer. Report it within 21 days of delivery with a photo; no need to send it back.

Full refund & replacement policy

The physics

When an X-ray bounces off a free electron, it comes away with a longer wavelength. Its speed is still c. It has handed some energy and momentum to the electron.

Arthur Compton measured the shift in 1923 and explained it by treating light as quanta with momentum h/λ: the shift depends only on the scattering angle, not on the incoming wavelength.

Comptonne's shot works the same way. The cue ball moves, the wave gets longer, and the light still travels at c. In the art, red stands in for the longer wavelength; Compton used X-rays.

Equations

\[\lambda' - \lambda = \frac{h}{m_e c}\left(1 - \cos\theta\right)\]
\[\frac{h}{m_e c} \approx 2.426\ \text{pm}\]
Symbols
SymbolMeaningUnit
\(\lambda, \lambda'\) wavelength before and after scattering \(\mathrm{m}\)
\(h\) Planck constant \(\mathrm{J·s}\)
\(m_e\) electron mass \(\mathrm{kg}\)
\(c\) speed of light \(\mathrm{m/s}\)
\(\theta\) scattering angle of the photon \(\mathrm{rad}\)
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