You plug it in. Nothing for a moment. Then a faint red-pink glow appears, like a neon sign firing up. That is exactly what it is: neon gas ionizing (640 nm, the same colour as a neon sign). The sodium is still sitting as solid metal on the tube wall, doing nothing. But the neon plasma heats the tube through conduction and ion bombardment, and the temperature slowly rises. Sodium melts at 98 °C, but only around 260 °C is there enough sodium vapor to overpower the neon. Then the shift happens. Pink patches turn yellow. Within a few minutes the neon light has disappeared entirely and the room is bathed in deep yellow. Sodium needs a fraction of the energy neon requires (2.1 eV vs. 16.6 eV), so once it is in the vapor phase, it captures nearly every electron. The neon is reduced to a supporting role: it carries the current, but all the visible light comes from sodium.

The light comes from a single atomic transition. Sodium's outermost electron gets kicked from the 3s orbital to the 3p orbital by a collision. After 16 nanoseconds it falls back and releases a photon at exactly 589 nanometers: yellow light. That is it. One wavelength. Because there is only one wavelength, objects can only reflect it or absorb it. A red shirt reflects no 589 nm light, so it looks black. A yellow sunflower does, so it stays yellow. Everything becomes shades of amber and shadow. The Color Rendering Index is -44: a negative score. No other commercial light source scores this low. (Technically it is a doublet: two lines 0.6 nm apart, caused by spin-orbit coupling. You cannot see the difference with your eyes.)