A photon falls

1. Apr 30, 2007

Gyroscope

1. The problem statement, all variables and given/known data
A photon of frequency f falls in a well x meters of height. What is the energy of the photon when it is at the bottom of the well.

The energy of a photon is hf. It would gain mgx for falling in the well, but since his mass is zero, the energy remains the same. Isn't it?

2. Relevant equations

3. The attempt at a solution

2. Apr 30, 2007

Dick

If you think about conservation of energy, it would clearly have to gain the same amount of energy as a mass of size hf/c^2. Otherwise, converting energy to mass, dropping it and converting back to energy would leave you with a puzzling balance sheet.