Question

Light from sodium lamp is passed through cold sodium vapours, the spectrum of transmitted light consists of

Answer 1

A sodium-vapor lamp is a gas-discharge lamp that produces light with a wavelength of around 589 nm by using sodium in an excited state. There are two types of these lamps: low pressure and high pressure. Although low-pressure sodium lamps are very efficient electrical light sources, their yellow light limits its usage to outdoor illumination, such as street lamps. Although high-pressure sodium lamps emit a larger spectrum of light than low-pressure sodium lamps, its colour rendering is still inferior to that of other types of lights. Low-pressure sodium lamps only emit monochromatic yellow light, which impairs nighttime colour vision.

Complete step by step answer:
To initiate the gas discharge, low-pressure sodium lamps feature a borosilicate glass gas discharge tube (arc tube) filled with solid sodium and a tiny quantity of neon and argon gas in a Penning mixture. The discharge tube can be either linear (SLI lamp) or U-shaped (U-shaped lamp). When the lamp is first turned on, it produces a faint red/pink light to warm the sodium metal; after a few minutes, the emission changes to a brilliant yellow as the sodium metal vaporises. These lamps emit a nearly monochromatic light with a wavelength of 589.3 nm on average (actually two dominant spectral lines very close together at 589.0 & 589.6 nm). It's difficult to identify the colours of things lit by this limited bandwidth. The spectrum of frequencies of electromagnetic radiation released by an atom or molecule transitioning from a high energy state to a lower energy state is known as the emission spectrum of a chemical element or chemical compound.
Kirchhoff's three laws of spectroscopy detailed the distinctions in these spectra as well as an explanation of how to produce them:
All wavelengths of light are emitted by a bright solid, liquid, or dense gas.
A BRIGHT LINE or EMISSION LINE spectrum is produced by a low density, hot gas viewed against a colder backdrop.
A DARK LINE or ABSORPTION LINE spectrum is created when a low density, cold gas is placed in front of a hotter source with a continuous spectrum.

Note:
The energy difference between the two states is equal to the photon energy of the emitted photon. Each atom has a number of different electron transitions, each with a different energy difference. An emission spectrum is a collection of various transitions that lead to different radiated wavelengths. The emission spectra of each element is distinct.