What is the de Broglie wavelength of an electron that strikes the back of the face of a TV screen at 1/9 the speed of light?

Answer 1

Calculate the momentum p of the electron using the classical formula,
p = m v = m c/9
m is the electron mass.
(It is OK to use the classical formula for momentum since the speed is much less than c. The correction factor is sqrt {1/[1 - (v/c)^2]} is close to 1)

For the de Broglie wavelength L, use

L = h/p