How do you work out the wavelegth (Franck-Hertz experiment)?

[birdnotbeing]

Homework Statement

In a Franck-Hertz experiment carried out with potassium vapour, it is found that the current falls off rapidly at an applied voltage of 1.62V. Calculate the wavelength of the expected spectral line in the emission spectrum of potassium when this voltage is reached.

Homework Equations

E = hf = hc/lambda

The Attempt at a Solution

delta E = 1.62 eV
lambda = hc/1.62 = 3.08x10^-25

According to a powerpoint, it should be 1.24/eV = 1.24/1.62 = 767nm (which is right). Where does 1.24 come from?

 

[Lord Jestocost]

Check the values and units of the constants (Planck constant, speed of light in vacuum) which you are using. The NIST Reference on Constants, Units and Uncertainty will be of help: https://physics.nist.gov/cuu/Constants/index.html