Why do we have sodium vapour lamp in the streets and not mercury vapour lamp?

            White light consists of seven colours – VIBGYOR. During foggy days air is highly humid and contains lots of tiny water droplets which can act like a prism. So, if white light is used in automobile headlights and street lamps, water droplets will split it into its component colours, thereby, creating a circular rainbow around each light source. This will affect clarity of vision and lead to accidents. In order to avoid this effect, a monochromatic (yellow) light source is used. Yellow is chosen because, being in the middle of the visible spectrum, it undergoes medium refraction and medium reflection while passing through a water droplet. Also, it is next only to white in brightly illuminating all objects in their original colour.

  However, mercury vapour lamps are used in the interiors because, white light is more pleasing to our eyes, and slight reduction of vision due to fog is not of great concern.

   The efficiency of sodium vapour lamps is good – between 40 and 50 lumens per watt. Hence they are used for street lighting.