I want to know the answer to the following question:
The energy difference between filled 3d and half-empty 4s atomic shells corresponds to what color of light?
All the background I'm provided with is that "The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells is such that it corresponds to orange light.". Is that enough to answer the question?
Answer: yes