What minimum speed must an electron have to excite the 492-nm-wavelength blue emission line in the hg spectrum?

The energy required by the excitation of the line is:
ΔE = hν = hc / λ
where:
ΔE = energy difference
h = Planck constant 
ν = line frequency
c = speed of light
λ = line wavelength

The energy difference must be supplied by the electron, supposing it transfers all its kinetic energy to excite the line:


Therefore, 


And solving for v we get:


Plugging in numbers (after trasforing into the correct SI units of measurement):

=9.4 · 10⁵ m/s

Hence, the electron must have a speed of 9.4 · 10⁵ m/s in order to excite the 492nm line.

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stands for electromotive force and it is measured in volts. despite its name, it is not actually the force in the sense of being a "pushing" or "pulling" (not measured in newtons).an equation that relates emf,  ℰ,  to a battery's terminal voltage, v, current, i, and internal resistance, r, is    ℰ = v + irin this case, the emf can be interpreted as a voltage—a difference in electric potential.i would not say that it would be considered an energy. if emf can be defined as the energy/work per a unit charge from a battery or other electric source, then it is not strictly energy.