Question

Why does a sodium vapor street light look yellow instead of white?

Answer 1

This is because these lamps have sodium vapor in them. When  the light from the tungsten filament hits the sodium vapour molecules, the sodium emits a yellow glow. This is the electromagnetic spectrum of sodium atoms. This is also why when you burn salt (Sodium Chloride) in a flame it burns with a yellow glow. Streetlight is made of sodium vapour becaue they use less electricity hence conserve energy.