## Conceptual Physics (12th Edition)

Published by Addison-Wesley

# Chapter 36 - Think and Explain: 33

#### Answer

The change in energy for light is demonstrated by a change in its frequency.

#### Work Step by Step

The energy of a photon is given by E = hf, where h is Planck's constant. If the energy of light is lowered, as in traveling upward in a gravitational field, its frequency is lowered (the light is said to be red-shifted). On the other hand, if the light's energy is increased, as in falling downward in a gravitational field, the frequency is raised (the light is blue-shifted).

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