Answer I: The light in Sodium Vapour lamp is from an atomic emission process whereas in Mercury Vapour Lamp it is, finally, from fluorescence emission. The mechanism of light emission in a sodium vapour lamp is simple and straight-forward. The filaments of the lamp sputter fast moving electrons, which hit the sodium atoms(vapour) causing the valence electrons of the sodium atoms to excite to higher energy levels and the electrons thus excited relax by emitting the characteristic monochromatic bright yellow light(589nm). The mechanism in mercury vapour lamp is more involved and sequential. The sputtered electrons from the filaments, after having been accelerated by high voltage, hit the mercury atoms. Here also, the excited electrons of mercury atoms relax by emitting characteristic but ultraviolet (254nm,invisible) light. The photons of this ultraviolet light fall on the fluorescent layer on the inner walls of the tube and excite the molecular bonds of the fluorescent material to various electronic and vibrational energy states. Hence, the light from the mercury vapour lamp is white.
Answer II: The basic difference between the two is, the former works by electric discharge (passage of electricity through sodium vapours at high/low pressure) while the latter works through the combined effect of electric discharge through mercury vapours and fluorescence from phosphors (luminescent materials). Although sodium vapour lamps produce much higher light output (about 90 lumens/watt) they cannot be used in lighting applications where colourrendering property is very crucial. This is because most of the light emitted from a sodium vapour lamp is concentrated in the yellow part of the visible spectrum (around 580-590 nm) On the other hand, a mercury vapour lamp is quite suitable for lighting applications. This is because; the mercury vapour lamp can feed almost the entire visible region (380-780 nm) of the human visual system. Conventional fluorescent lamps can also be called as low-pressure mercury discharge lamps. In this system, when electric discharge strikes mercury vapours held at low pressure ( a few mm of mercury) it produces a lot of ultraviolet radiation dominantly at 254 nm inside the column of the discharge tube. This UV radiation when impinging on the white coating made of fluorescent materials coated inside the discharge column of the tube will generate white light (called daylight). Light output from a fluorescent lamp is moderate (60 lumens/watt) while the color-rendering index is high.