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

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.