What does four times the accelerating voltage do?

Electrons start effectively at rest and are accelerated through a potential difference of magnitude V. In a second run, the accelerating potential is 4V.

What is the ratio of the second de Broglie wavelength to the first?

Organize the givens & assumptions

First kinetic energy: eV
Second potential: 4V
Same particle mass

Both final speeds are nonrelativistic. Ignore initial kinetic energy and collisions. Use the electron-charge magnitude in the energy gain.

A $\tfrac14$
B $\tfrac12$
C $2$
D $4$

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