This is one of the questions on my regular worksheets we have before our tests: A fast-moving electron travelling through a vacuum tube slams into a piece of steel, coming abruptly to rest and emitting an X-ray photon with an energy of 7.90×10-16 J. What is the wavelength of the photon? I know I have to find the frequency and speed of the photon in order to find the wavelength. The problem is I do not know how to find either of the two (I had a long day at work so I'm pretty sure I'm just having a brain fart right now). If someone could help point me in the right direction it would be more than appreciated!