Neon (Ne) is a noble gas that is colorless, odorless, and inert under standard conditions. It is the fifth most abundant element in the universe. Neon's most familiar use is perhaps in neon signs, where it produces a characteristic bright reddish-orange glow when electricity is applied. This luminescence in neon lighting results from the excitation of neon atoms, which then release a photon as they return to their ground state. Additionally, due to its inertness, neon is used in some types of gas lasers, such as helium-neon (He-Ne) lasers, where it serves as the lasing medium to produce a coherent red light. The low refractive index of neon gas is also of interest in certain optical applications.