Question

Why does a sodium street lamp give off a yellow color whereas a neon light gives off a red color?

Answer 1

The yellow light from the Na lamp and the red color of the Ne lamp are due to electronic de-excitations each of which are associated with characteristic wavelengths

Step-by-step explanation:

The color of light given off depends on the energy difference between the two levels involved in electronic transition. In general, energy is absorbed when electrons are excited from a lower to a higher energy level whereas energy is released in the form of photons or light when electrons fall back from a higher to a lower level.

In the case of a Sodium (Na) street lamp the yellow color falls around 589 nm (yellow region of the visible light) as is due to electron de-excitation from the 3p to the 3s level.

Similarly, in the case of a Neon (Ne) street lamp the red color falls around 632 nm (red region of the visible light) as is due to electron de-excitation from the 5s to the 3p level of the Ne atom.

Answer 2

The color of the light depends on the element to which it interacts. On interacting with energy, the electrons goes to the higher excitation state and when electron drops to lower state which is more stable, the energy is re-emitted as light. Each element produces its own characteristic color.

In case of a sodium street lamp, more photons in the long wavelength are emitted by the sodium gas, leading to yellow color one see. Whereas in case of a neon light, the neon gas is a noble gas, stable and non-reactive. The electrons in neon will excite on applying current. The free electrons will collide with each other and then return back to the atoms. The energy is produced in the form of light on absorbing electrons by the atoms. This falls in the red region of spectrum and thus emits red color.