# Chapter 31 - Think and Explain: 28

The first equation is a proportion, and the second equation shows that the constant of proportionality is h, Planck's constant.

#### Work Step by Step

For photons, E = hf, where E is the energy, h is Planck's constant, and f is the photon's vibrational frequency. In other words, h is the proportionality constant that relates the energy and the frequency of a photon.

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