KnowHOW team explains: In an atom, particles known as electrons revolve in different orbits. These orbits are fixed. An electron possesses a certain amount of energy to maintain its position in a given orbit.

Electrons, however, can change orbits. When one of them jumps to another orbit, it either absorbs or releases an amount of energy so that it can fit itself well in the new path. When an electron jumps back to an inner circle, it releases energy which manifests itself as light of a particular colour, because colour, as we know, is the sign of a distinct wave length of the light we see.

When electricity (usually of a high voltage) is passed through a gas confined in an air-tight enclosure ? preferably a glass tube ? the electrons present in the atoms of the gas absorb a portion of electrical energy which is sufficient to knock them off their individual orbits. Soon after, these electrons release that ‘extra’ energy in the form of light of a particular colour and jump back to their orbits. This goes on as long the electricity passes through the gas.

When the gas is neon, the light emitted assumes a red-orange colour. However, for producing other colours many other gases including argon, helium, krypton and xenon are used. So, the glass tubes of the neon signs glow in different colours because they contain gases other than neon.

The question was sent by P. Anantha from Jamshedpur