What is the Emission Spectrum?

Every state of matter tries to be at minimum potential energy or it can be said that the atoms of element/ substance arrange themselves such that overall energy is minimum.

Whenever atoms and molecules of elements interact with electromagnetic radiation, this excites the outer electrons and they jump to higher energy orbits. This makes the atom and molecules of the element unstable. With time, to reach back to the initial state i.e. to go over to a more stable state, the electrons move from high energy orbitals to lower orbitals. During this process the atoms and molecules radiate energy which correspond to different regions of the electromagnetic spectrum. This spectrum of radiation which is emitted by the electrons when they are in an unstable excited state is known as emission spectrum.

Concept

The emission spectrum is, thus defined as the spectrum of frequencies of the radiations emitted by an atom or molecules when the element makes transition from higher energy level to the lower energy level.

The electrons inside the atom are usually structured in a quiet manner so that the atom's energy is as limited as feasible. The ground level of an atom is its lowest energy state. As certain atoms gain energy, the electrons absorb this energy and jump to a higher energy level. Such electron energy levels in atoms are mapped, this infers that the electron must travel between one energy level to the next in sequential steps instead of just gradually. An atom's excited level is the one which has greater energy than the ground level. The excited state of the atom isn't really stable. So to attain stability it would drop down to the ground level. During this process it releases the energy it had absorbed, as radiation.

How do Atoms Gain Energy?

One way is through passing electricity through low pressurized gas of the given element. Every element has their own distinct electron energy. It is like having an independent trait of every element. In a discharge tube gaseous samples of every element will glow in very distinctive colors.

Atomic spectra:

When a beam of white light strikes a mirror, it undergoes refraction twice. Formerly as it passes from a rarer medium (air) to a denser medium (glass), and once more when it travels from a denser medium (glass) to a rarer medium (air) (air). Ultimately, we see a band of colors known as the rainbow, which is created by a beam of white light. When we examine this band of light more closely, the color with the shortest wavelength shifts away the most, and vice versa. As a result, a spectrum of colors ranging from red to violet is observed, with red suffering the least variation because of its longest wavelength. This type of spectrum is known as a continuous spectrum because violet blends into blue, blue into green, and so on. The atoms in the gas phase have an emission spectrum which does not show a constant spread of wavelength from one color to the next. Rather, the released light is made up of a single spectrum with dark spaces in between. These spectra are known as atomic spectra or line spectra.

Absorption spectrum:

Unlike the emission spectrum, an absorption spectrum can be considered as a photographic negative of the emission spectrum. Electromagnetic radiation is bombarded on a sample that receives radiation of specific wavelengths in order to observe the absorption spectrum. The wavelength of radiation absorbed by matter corresponds to those wavelengths which were lost, leaving dark gaps in the otherwise light continuous spectrum. Every unit has a distinct line emission spectrum. Spectroscopy is the analysis of the light spectrum or absorption spectrum.

Molecular Spectrum:

Aside from spectra correlated with atoms and ions, molecules may interfere with electromagnetic radiation and produce distinct spectra. Because of their simple atomic and molecular composition, molecules' spectra usually include infrared wavelengths. Furthermore, since molecules are typically fragile, molecular spectra are most significant in objects that are relatively cold. Furthermore, since molecules are typically delicate, molecular spectra are most significant in comparatively cold structures such as planetary atmospheres, the surfaces of very cool stars, and various interstellar regions.

Hydrogen spectrum:

When an electric current is made to pass into a glass tube containing hydrogen gas at  low-pressure, the tube emits blue light. As this light is made to pass through a prism , four small bands of bright light with a black backdrop are observed. This hydrogen spectrum is especially significant in astronomy because hydrogen constitutes the majority of the Universe. Series are sets of lines leading to atomic transitions, each ending or starting of the same atomic state of hydrogen as a result of hydrogen emission or absorption processes.

Thus, the Balmer Series consists of transitions that begin (for absorption) or end (for emission) with the first excited state of hydrogen, while the Lyman Series consists of transitions that begin or end with the ground state of hydrogen; the accompanying picture depicts the atomic transitions that generate these two series of emission.

The idea that only a limited number of frequencies can be emitted or absorbed by the hydrogen atom means that these atoms only absorb radiation which corresponds to a certain few energies. This indicates that the hydrogen atom has a limited number of energy values. These measurable energy levels correspond to the quantized energy levels of the hydrogen atom.

When a photon is captured by a hydrogen atom, the energy of the absorbed photon makes the electron move to a higher energy level (n=1, n=2, for example). When a neutron is absorbed by the hydrogen atom, the electron moves from a higher to a lower energy frequency (n=3 n=2, for example). There is light propagation during this transfer from a higher level to a lower level. Since the atom's energy levels are quantized, the spectrum will consist of wavelengths corresponding to the energy changes between these energy levels. The line at 656 nm, for example, corresponds to the transformation n= 3, n=2.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for      

• B.Sc.
• M.Sc.
• M.tech.