Emission Spectrum

Definition of Spectrum 

Before we discuss the emission spectrum definition, let us address the questions - what is a spectrum in chemistry and what is a spectrum in physics. Whether it is physics or chemistry, the spectrum definition is the same - when white light is passed through a prism or any other dispersing substance, the white light splits into a series of coloured bands or lines known as a spectrum. The different constituent wavelengths of white light are arranged in the spectrum in a specific order, starting with the longest wavelength (red) and shading through to the shortest (violet).

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What Is Emission Spectrum?

So, what is the emission spectrum definition in physics and chemistry? An emission spectrum is the range or array of wavelengths (spectra) obtained when the light emitted by a substance is passed through a prism and examined directly with a spectroscope.

Now let's define the line emission spectrum: a spectroscope splits the emitted light into different wavelengths and gives a discontinuous spectrum in the form of discrete lines known as a line spectrum. An example of an emission spectrum is when copper is heated on a flame, and the flame gets green color.


Production of Emission Spectrum

When an atom or molecule absorbs energy, the electrons are excited to a higher energy level. When the electron falls back to the lower energy level, light is emitted, which has the energy equivalent to the higher and the lower states’ energy difference. Due to the availability of multiple states of energy, an electron can undergo numerous transitions, each giving rise to a unique wavelength that comprises the emission spectrum. 


Atomic Spectra

We know that when elements or their compounds are heated, they release energy in the form of light, which gives rise to a line spectrum. However, when atoms in their elemental form are heated or excited, the line spectra that originates are known as the atomic spectra.


Absorption Spectrum

When electromagnetic radiation passes through a material, a part of the electromagnetic radiation may be absorbed. In that case, when the remaining radiation is passed through a prism, a spectrum is obtained with a gap in it, called an absorption spectrum. The absorption spectrum is characteristic of a particular element or compound and does not change with varying concentrations.

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Hydrogen Emission Spectrum

When the hydrogen atom gets energy from outside, its electron goes from the lowest energy level to some higher energy level. But it returns from there, within 10-8 seconds. To the lowest energy level directly or via other lower energy levels. While returning, the electrons emit light. 

Suppose two energy levels of the hydrogen atom, n1, and n2, have energies E1 and E2, respectively. If the electron's transition takes place from the higher energy level n2 to the lower energy level n1, it will emit a photon of light of energy (E2 - E1). The frequency 'ν' of the emitted light is given as:

ν = R [(1/n12) - (1/n22)] where, ‘R’ is the Rydberg constant.


Hydrogen Transitions

  • Lyman series: An electron on returning from some higher energy level to the first energy level (that is, n1 = 1 and n2 = 2, 3, 4, etc.), then the emitted series of spectral lines are obtained in the ultraviolet region.

  • Balmer series: An electron on returning from a higher energy level to the second energy level (that is, n1 = 2 and n2 = 3, 4, 5, etc.), the emitted spectral lines are obtained in the visible region.

  • Paschen series: An electron on returning from some higher energy level to the third energy level (that is, n1 =3 and n2 = 4, 5, 6, etc.), then the emitted lines are obtained in the infrared region of the spectrum.

  • Brackett series: An electron on returning from some higher energy level to the fourth energy level (that is, n1 = 4 and n2 = 5, 6, 7, etc.), the emitted lines are obtained in the infrared region of the spectrum.

  • Pfund series: An electron on returning from a higher energy level to the fifth energy level  (that is, n1 = 5 and n2 = 6, 7, 8, etc.), the emitted lines are obtained in the infrared region of the spectrum.

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FAQ (Frequently Asked Questions)

1. What is the Difference Between the Absorption and Emission Spectrum?

  • An absorption spectrum is defined as the spectrum obtained when electromagnetic radiations are passed through a substance; a part of the radiation is absorbed by the material, and the rest is transmitted. An emission spectrum is defined as the spectrum observed when electromagnetic radiations are given off by a substance.

  • An absorption spectrum is observed when atoms absorb some energy. But the emission spectrum is a result of atoms releasing energy.

  • An absorption spectrum is characterized by dark lines or gaps, while an emission spectrum typically shows colored lines.

  • When an atom gives an absorption spectrum, it is because it has gained a higher energy level. In contrast, an emission spectrum results when an atom falls back to a lower level from an excited state with the release of energy.

  • Absorption spectrums account for the wavelengths that a substance absorbs. The emission spectrum accounts for the emitted wavelengths. 

2. What are the Applications of Emission and Absorption Spectra?

  • Absorption spectroscopy studies radiation absorbed at various wavelengths. When electromagnetic radiation passes through a sample, most of it passes through the sample without loss in intensity. At specific wavelengths, however, the radiation's energy is attenuated; this is known as absorption. Absorption spectroscopy gives qualitative as well as quantitative information about the sample.

  • Atoms and molecules can be excited to high energy levels, and when they fall back to the lower levels, radiation is emitted in the form of light. When atoms are excited by high temperature, the light emission is called atomic emission. This principle is used in emission spectroscopy to study the structural details of atoms and molecules. For atoms excited by electromagnetic radiation, the light emission is called atomic fluorescence; it is used in fluorescence spectroscopy for analytical purposes in various scientific fields.