# Franck–Hertz experiment question

Greetings fellow Humans.

I have been given a problem as the following. In a Franck–Hertz experiment, the jumps in Voltage in the characteristic Curve are U=2.104 volts.

a) What color does the Gas glow?

b) What Element could the Gas be?

c)How big must the electron speed be so that it makes the Gas glow through Collision only?

As far as a) goes, it is logic that in the Franck–Hertz experiment, when the electrons are accelerated by a Voltage, they gain energy as e*U, in our case 2.104 eV. This would mean that once the Gas atom has gained this much energy from the electron, it will go in an excited state and decay again releasing a photon with this same energy.

This would give us a 2.104 eV photon, which corresponds to 589.28 nm of Light-Wavelength. (Orange color).

What i would need help with are the b and c tasks, or at least a hint as to where to start them from. For c) for example, i want to go with the logic that if the gas has to glow through collision only, it has to be able to reach the draper point, we calculate the Temperature of the gas and then handle the electron as a point particle with mass and kinetic energy (hence speed).

Thoughts would be very appreciated.

• Nickel has a spectral line at 589.28nm. Google is your friend -- use the key words "atomic spectra 589.28nm". For (c) you should start by calculating the speed of the electron. – Semoi Jun 21 at 7:06