JEE Main · 2021 · 24 Feb 2021 · Shift 2 · s-Block Elements

The flame colour of an alkali metal salt comes from light emitted when the thermally excited valence electron falls back to its ground state, and each metal emits at its own characteristic wavelength. Lithium emits crimson light at $670.8$ nm, sodium golden yellow light at $589.2$ nm, rubidium red-violet light at $780.0$ nm and caesium blue light at $455.5$ nm. So LiCl goes with $670.8$ nm, NaCl with $589.2$ nm, RbCl with $780.0$ nm and CsCl with $455.5$ nm.