# Interactions of Light and Matter

1. Oct 2, 2009

### EskShift

1. The problem statement, all variables and given/known data
The tungsten cathode in an electron microscope has a work function of 4.5 eV and is heated to release electrons that are initially at rest (near the cathode). The electrons are accelerated by a potential difference that creates a beam with a de Broglie wavelength of 0.040 nm.

Calculate the kinetic energy, in Joules, of an electron in the beam.

2. Relevant equations
Ek = hf - w

F = c / $$\lambda$$

3. The attempt at a solution
Ek = hf - w
F = c / $$\lambda$$
Therefore, Ek = h x (c/$$\lambda$$) - w
Ek = 6.63 x 10^-34 x (3 x 10^8/0.040 x 10^-9) - 4.5
Ek = -4.5?
Feel like im doing something very wrong here...

2. Oct 2, 2009

### Staff: Mentor

How does the de Broglie wavelength relate to the particle's momentum?