Which of the following gases emits red light when electricity is passed through it?

Correct — B. Pass electricity through neon at low pressure and it glows a strong red-orange.

The colour comes from neon's own emission lines, which cluster at the red end of the visible spectrum. Every gas has its own set, so every gas has its own glow.

That red is the reason advertising tubes came to be called neon signs at all, and the keyed gas is option (b).

Why the others are wrong

• (a)
  Argon — Argon glows lilac to pale violet-blue in a discharge tube. Its exam association is shielding and filling work, not red light.
• (c)
  Hydrogen — Hydrogen gives a pink-magenta discharge: its bright red line is joined by blue and violet lines, so the tube does not read as pure red.
• (d)
  Helium — Helium discharges look pale peach or whitish-pink — much weaker and much less red than the glow the question describes.

An excited gas emits only at the wavelengths its atoms allow — a line spectrum unique to that element.

In a discharge tube the current excites the atoms, they fall back to lower energy levels, and the tube shines with the element's own set of colours. Neon's strong lines lie in the red-orange, so a neon tube is red.

This is the same physics that lets an astronomer name an element in a distant star from its spectrum alone.

Key facts

• Neon is a noble gas of group 18 with atomic number 10.
• Neon in a low-pressure discharge tube emits a characteristic red-orange light.
• Each element emits a unique line spectrum, which fixes the colour of its discharge tube.
• Argon is used as a shielding gas in welding rather than for red light.

Common traps

• Helium and neon sit next to each other in the same group, so the two get swapped.
• Assuming every coloured advertising tube contains neon, when the word has become a label for the whole family of discharge signs.

No directly related past PYQ was found.