What is an optically active electron?

[Filipa]

I was studying the spin-orbit interaction and the Zeeman effect, and came across the concept of optically active electrons.
Initially I got the idea that an optically active electron is any unpaired electron. But then, while trying to understand the Zeeman effect in a Cadmium atom, for which all electrons in the ground state are paired, I got confused because for this atom there are optically active electrons, otherwise it wouldn't be observable the Zeeman effect.

So, can anyone clarify to me what actually is an optically active electron?
Thanks in advance

 

[gr71cj5]

"Turn on the power supply to the cadmium lamp. Allow 5 minutes for the red cadmium line to be sufficiently intense"
https://www.physics.nus.edu.sg/~L3000/Level3manuals/zeeman effect.pdf
From your question: "trying to understand the Zeeman effect in a Cadmium atom, for which all electrons in the ground state are paired"
Cadmium only exhibits optically active electrons when excited (heated in this case).
"The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state" https://en.wikipedia.org/wiki/Ground_state