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.

It may possibly be c i think |-/