Why is the luminous efficiency small for a filament bulb as compared to a mercury vapour lamp ?

The yellow colour has a greater luminous efficiency as compared to the other colours. Can we increase the illuminating power of a white light source by putting a yellow plastic paper around this source ?

The overall luminous efficient of a 100 W electric lamp is 25 lumen W^-1 . Assume that light is emitted by the lamp only in the forward half, and is uniformly distributed in al diredctionsin this half. Calculate the luminous flux falling on a plane object of area 1cm^2 placed at a distance of 50 cm from the lamp and perpendicular to the line joining the lamp and the object.

Small drops of mercury are found to assume a spherical shape. Why?

The luminous flux of a 1 W sodium vapour lamp is more than that of a 10 kW source of ultraviolet radiation. Comment.

A red flower seen in the light of mercury vapour lamp through a green filter will appear

Mercury vapour lamp : ________ :: Sodium vapour lamp : monochromatic light