Why we use mercury vapour lamp, arc light or sunlight for photochemical reaction???

Mercury-vapor lamps are light sources with discrete, polychromatic emission (type B). Depending on the pressure used, mercury-vapor lamps can also show a continuous, polychromatic emission. The same applies for certain phosphor coatings for fluorescence lamps. A discrete emission tends to be beneficial, since filtering of undesired wavelengths can be realized easier. The optical properties of band-pass filters can be less demanding. The geometrical properties of light sources of type B are usually similar to that of type A. Light emission in most cases is circular. Despite the differences in the shape of the lamp tubes, this also applies for CFLs, as long as the radiation field outside the entire light source is considered.