Question

What causes the emission of radiant energy that produces characteristic spectral lines ?

Answer 1

The energy of electromagnetic waves is known as radiant energy.These waves have the ability to fly across space. Electromagnetic radiation is made up of tiny particles called photons, which can be thought of as energy packets. The human eye can see light energy, which is a form of radiant energy.

Complete answer:
The radiant energy release was accompanied by the hop of electrons between allowed orbitals.The Hydrogen Atom is a simple example to consider. When a hydrogen Atom receives enough energy, the electron will hop from the ground state to the excited state. Let's take a look at the hydrogen atom from the Bohr model's eyes. Assume that a ray of white light (photons of all visible wavelengths) passes through an atomic hydrogen gas.

A photon with a wavelength of $656$ nanometers has only enough energy to lift an electron from the second to the third orbit of a hydrogen atom. As photons of various energies pass through the hydrogen atoms, photons of this wavelength can be absorbed by atoms with electrons orbiting on the second stage. The electrons on the second level will pass to the third level as they are absorbed, and a number of photons of this wavelength and energy will be absent from the general stream of white light.

Consider a container of hydrogen gas through which a stream of photons passes, allowing several electrons to travel to higher levels. When the light source is turned off, these electrons "fall" back down from larger to smaller orbits and emit photons of light—but only light of the energies or wavelengths that correspond to the energy difference between allowable orbits. This orbital changes of photons of hydrogen atoms from higher level to lower level give rise to the spectrum lines and the release of a huge amount of radiant energy.

Note: Our atom's emission spectrum will be composed of radiation (a photon of energy equal to the energy gap between the two orbitals and given as $E = Hf$ where $f$ is the frequency and $H$ is Planck's Constant). Atomic spectra is the study of atoms (and atomic ions) through their interaction with electromagnetic radiation.