Emission and Absorption of Light

Origin of Emission and Absorption Process

In the mid 19th century, scientists found that, when the materials are heated in flames or kept in electrical discharge, the lights are emitted with well-defined frequencies. The study of emission and absorption spectra of atoms plays a predominant role in developing the atomic structure. Many attempted and failed to describe the origin of the emission and absorption process of light with a simple atom and hydrogen framework using electromagnetism and mechanics. 

Bohr’s Atom Model  

In 1913, A Danish physicist, Neil Bohr proposed the hydrogen atom model. It explains the structure of atoms, especially the hydrogen atom. The electrons orbiting around the nucleus in the atom allowed certain stationary states with well-defined energies. When an electron transits from one state to another, the atom can absorb or emit photons.  The energy conserved during this process is determined by the frequency of the light emitted or absorbed.  The entire process depends on the emission and absorption process of light. 

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When the electrons are excited and returning from the higher energy state to the lower energy state, the light with certain frequencies will get emitted. 

Spontaneous Emission

Spontaneous emission is the process of a quantum mechanical system, which transits electrons from an excited state to a lower energy state and emits a quantized amount of energy in the form of a photon. 

If the isolated atom is excited from the lower energy state to the higher energy state, the atom will remain the same for a short period, before emitting a photon. On average, when the atom excites, the average time for the spontaneous emission of photons will stay in the order of  10−9 to 10−8 seconds.

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The electrons in an excited state will automatically reach the lower state during spontaneous emission. Stimulated emission requires some external energy to stimulate it back to lower energy levels. 

Stimulated Emission  

In the stimulated emission, the photons with certain energy will trigger an atom in an excited state to emit a photon to transit to a lower energy state. 

According to the intensity of the light falling on the atom, the atom will absorb a photon and excites, this process is known as a probabilistic event. Even without knowing the principle behind stimulated emission, Einstein in 1917, presented the thermodynamic arguments, which contains three types of radioactive transitions in an atom-stimulated emission. Here, the energy released during the stimulated emission is directly proportional to the intensity of the light intense atom. 

Einstein expressed that the three possible radiative transition routes depend on emission and absorption processes, They are also called Einstein coefficients. They are

  1. Spontaneous emission

  2. Stimulated Emission

  3. Absorption 

According to Einstein’s stimulated emission process, the energy of the emitted photon is equivalent to the stimulating photons. The energies get polarized in the same direction, in phase with photons. After 40 years of working with Stimulated emission, Einstein invented the LASER (Light Amplification by Stimulated Emission of Radiation). The laser lights will work on the property of stimulated emissions.  The Laser lights are highly monochromatic, directional, and coherent.  


Spectroscopy is the specific study of colours from the visible range of the electromagnetic spectrum.  The study of emission and absorption of lights is known as spectroscopy. 

Applications of Spectroscopy 

We are using spectroscopy in our day-to-day life in various fields. Some of the applications include: 

  1. To determine the atomic structure of the samples

  2. You can find the oxygen content in the marine and freshwater 

  3. Can analyze the metabolic structure of muscles 

  4. Can be used as a respiratory gas analysis in the medical field

  5. Can use spectroscopy to improve efficiency through altering the structure of drugs 

Quantum Electrodynamics 

In the late 1920s and 1930s Paul Dirac, Pascual Jordan, Werner Heisenberg, Wolfgang Pauli, and others laid the foundation of the quantum mechanical theory of light. Richard Feynman, Tomonaga Shinichiro, and Julian S Schwinger together completely developed the theory of quantum electrodynamics. Quantum Electrodynamics explains the interactions of electromagnetic radiations with charged particles and among the charged particles.  In Quantum Electrodynamics, the photons will act as carriers of electric and magnetic forces. For eg, the two identical charged particles will electrically repel each other because of interchanging the virtual photons. It is impossible to measure the virtual photons directly. The existence of virtual photons will violate the conservation laws of energy and momentum. Here, every charged particle will emit the photons when it detects the light. 

Though the mathematical complexity of QED is not proven, the QED’s are theoretically proven through experiments. So, this is considered the prototype field theory of physics.  

The theoretical framework of the QED has involved the process of transformation of photons into matter and matter into a photon. During the creation of pairs, the photons will enter an atomic nucleus and it will disappear,  Further, its energy will be converted into electrons and positrons. During the pair annihilation, the electron-positron pair will disappear and high-energy photons are created. These processes are known as the central importance of cosmology, which explains that light is a primary component of the universe!! 

FAQs (Frequently Asked Questions)

1. How are Emission and Absorption Lines Formed?

Ans: The absorption lines are formed when the photons are absorbed by the electrons to transfer into the excited state from the lower energy state. When the electrons are transferring from the excited state to the lower one and photons are emitted.  During this emission of photons, emission lines can be absorbed.   Both the absorption and emission lines will have a different range of wavelengths on the continuous spectrum. 

2. What is the Difference Between Absorption and Emission Spectroscopy?

Ans: The absorption spectrum and emission spectrum will have different colour ranges.  

Absorption Spectroscopy 

Emission Spectroscopy 

It will be emitted when an atom absorbs energy, 

It will get emitted when atom releases energy

Contains dark lines in a spectrum 

Contains a wide range of colour lines in a spectrum

The wavelength of the light absorbed will help to find several substances in the sample. 

It is possible to figure out the kind of elements. Since the different elements will emit different energy radiations.