Photon energy is the energy of a single photon (light particle). You can calculate it using the wavelength of light \( \lambda \). For example, if the light has the wavelength \( \lambda = 550 \, \mathrm{nm} \), then the energy of a photon is:\begin{align}
W_{\text p} &= 6.6 \cdot 10^{-34} \, \mathrm{Js} ~\cdot~ \frac{ 3 \cdot 10^{8} \, \frac{\mathrm m}{\mathrm s} }{ 550 \cdot 10^{-9} \, \mathrm{m} } \\\\
&= 3.6 \cdot 10^{-19} \, \mathrm{J}
\end{align}

Wavelength

\( \lambda \) Unit \( \mathrm{m} \)

Wavelength of the electromagnetic radiation. Radio waves, for example, have a long wavelength. X-rays, for example, have a short wavelength.

Planck's constant

\( h \) Unit \( \mathrm{Js} \)

Planck's constant is a physical constant from quantum mechanics and has the value:$$ h ~=~ 6.626 \, 070 \, 15 \,\cdot\, 10^{-34} \, \mathrm{Js} $$

Speed of light

\( c \) Unit \( \frac{\mathrm m}{\mathrm s} \)

Speed of light (in vacuum) is a physical constant and has the value:$$ c ~=~ 3 \cdot 10^8 \, \frac{\mathrm m}{\mathrm s} $$

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