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Absorption Spectrum - Spectra and Spectroscopy, Emission Spectra and Absorption Spectra

Absorption Spectrum - Spectra and Spectroscopy, Emission Spectra and Absorption Spectra

Vishal kumarUpdated on 02 Jul 2025, 05:32 PM IST

Light is an electromagnetic wave and it has a frequency and a wavelength. This wave can travel in a vacuum at the speed of light. The entire range of the electromagnetic spectrum is given by radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays. The interaction of electromagnetic waves with matter causes the molecules and atoms present in a matter to absorb energy and reach a higher energy state. They have to emit energy in the form of radiation to return to their original state as the higher energy state is unstable and this process produces absorption spectra and emission spectra.

This Story also Contains

  1. Energy State
  2. Quantized Energy
  3. What Is Spectrum?
  4. Absorption Spectrum
  5. Applications Of Atomic Absorption Spectroscopy
Absorption Spectrum - Spectra and Spectroscopy, Emission Spectra and Absorption Spectra
Absorption Spectrum

Energy State

We know that energy is conserved. This means it can not be created or destroyed. Whenever an electron loses energy, a photon is created with the exact amount of energy lost. Whenever an electron gains energy, a photon with that amount of energy is absorbed. If electrons absorb energy, they can jump from a lower energy state to a higher energy state. On the contrary, when it transits from a higher energy state to a lower energy state, they emit energy. So, absorption or emission takes place when electrons transit from one energy level to another. We can find each possible photon’s energy using the electron energy levels.

Commonly Asked Questions

Q: What is the significance of the fine structure in atomic absorption spectra?
A:
Fine structure refers to the splitting of spectral lines due to the interaction between an electron's orbital angular momentum and its spin. This splitting provides information about the electronic structure of atoms and has been crucial in developing and testing quantum mechanical models of the atom.
Q: How do absorption spectra change in the presence of electric fields (Stark effect)?
A:
The Stark effect describes the splitting or shifting of spectral lines in the presence of an electric field. This effect can provide information about the electric dipole moments of atoms or molecules and is used in various spectroscopic techniques to study molecular structure and interactions.
Q: What is the concept of forbidden transitions in absorption spectroscopy?
A:
Forbidden transitions are those that violate selection rules based on quantum mechanical principles. While they are less likely to occur, they can sometimes be observed in absorption spectra, especially in astrophysical contexts. These transitions provide valuable information about atomic structure and environmental conditions.
Q: What is the importance of the ultraviolet-visible (UV-Vis) region in absorption spectroscopy?
A:
The UV-Vis region is important for studying electronic transitions in atoms and molecules. Absorption in this region can reveal information about molecular orbitals, conjugated systems, and transition metal complexes. UV-Vis spectroscopy is widely used in chemistry, biochemistry, and materials science.
Q: How do absorption spectra change in high-energy astrophysical environments?
A:
In high-energy astrophysical environments like accretion disks around black holes, absorption spectra can be dramatically altered. Extreme temperatures and strong gravitational fields can lead to broad, shifted absorption lines. Studying these spectra helps astrophysicists understand the physics of these extreme environments.

Quantized Energy

Electrons can not exist between energy levels in an atom. The energy of an electron is quantized means it can only exist in certain specific quantities, and not between those quantities. We can use electron energy level diagrams to find all possible energies of emitted and absorbed photons. Absorption and emission spectra are evidence for the fact that electron energy levels are quantized.

Commonly Asked Questions

Q: How do absorption spectra relate to the quantum nature of atoms?
A:
Absorption spectra directly reflect the quantized energy levels in atoms. The discrete lines in an absorption spectrum correspond to specific electron transitions between these quantized energy states, providing experimental evidence for the quantum mechanical model of the atom.
Q: What is the significance of the Balmer series in hydrogen's absorption spectrum?
A:
The Balmer series is a set of spectral lines in the visible region of hydrogen's spectrum, representing electron transitions to or from the n=2 energy level. It's significant because it's easily observable and was crucial in the development of atomic theory and quantum mechanics.
Q: How do absorption spectra change for different phases of matter (solid, liquid, gas)?
A:
Absorption spectra change significantly across different phases. Gases typically show sharp, distinct lines. Liquids often display broader absorption bands due to molecular interactions. Solids can show even broader bands or continuous absorption regions due to the formation of energy bands in the solid structure.
Q: What is the concept of oscillator strength in absorption spectroscopy?
A:
Oscillator strength is a measure of the probability of an atomic or molecular transition. It relates to the intensity of spectral lines and is important for quantitative analysis in spectroscopy. Higher oscillator strength means a stronger absorption line for a given concentration of absorbing species.
Q: How do absorption spectra change under extreme conditions like those in neutron stars?
A:
Under extreme conditions like those in neutron stars, absorption spectra can be dramatically altered. Intense magnetic fields can cause Zeeman splitting of spectral lines, while extreme gravity can lead to gravitational redshift. These effects provide valuable information about the extreme physics of such objects.

What Is Spectrum?

The band of colours obtained by passing light through a prism is called a spectrum. A rainbow and a prism are two ideal examples to describe the spectrum of radiation. The instrument which we use to separate the radiations of different wavelengths is called Spectrograph or Spectroscope and the branch of science which deals with the study of the spectrum is known as spectroscopy. The spectra obtained from different substances can be classified into two types:

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  • Absorption Spectrum: An absorption spectrum can be defined as a spectrum obtained by transmitting electromagnetic radiation through a substance. It is the spectrum of radiation emitted by a substance that has absorbed the energy. It is produced when atoms absorb energy.

  • Emission Spectrum: An emission spectrum can be defined as a spectrum of the electromagnetic radiation emitted by a substance. This spectrum is produced when atoms release energy.

When white light passes through a prism, it breaks up into seven colours. They are violet, indigo, blue, green, yellow, orange and red known as VIBGYOR. This we can see as a band of colours. The spectrum which is produced by splitting white light is continuous in the sense there are no gaps between these colours. So, we can say that the band of these seven colours is known as the spectrum. As this spectrum consists of different shades of each of these colours without any gaps, such a spectrum is called a continuous spectrum.

Commonly Asked Questions

Q: What is the difference between continuous and line spectra?
A:
A continuous spectrum contains all wavelengths of light without interruption, like a rainbow. A line spectrum, whether emission or absorption, shows distinct lines at specific wavelengths, indicating the presence of specific elements or molecules.
Q: What is the role of selection rules in absorption spectroscopy?
A:
Selection rules in absorption spectroscopy determine which transitions between energy levels are allowed or forbidden. These rules are based on quantum mechanical principles and help explain why certain absorption lines appear or are absent in a spectrum, providing insights into atomic and molecular structure.
Q: How do molecular absorption spectra differ from atomic absorption spectra?
A:
Molecular absorption spectra are more complex than atomic spectra. While atomic spectra show distinct lines, molecular spectra often display bands due to the additional energy states associated with molecular vibration and rotation. These bands can provide information about molecular structure and composition.
Q: What is the significance of the Fraunhofer lines in the solar spectrum?
A:
Fraunhofer lines are dark lines in the solar spectrum, first observed by Joseph von Fraunhofer. These lines represent absorption by elements in the Sun's outer layers and Earth's atmosphere. They were crucial in the development of spectroscopy and provided early evidence of the chemical composition of the Sun.
Q: What is the significance of the Lyman series in hydrogen's absorption spectrum?
A:
The Lyman series is a set of spectral lines in the ultraviolet part of hydrogen's spectrum. It represents electron transitions to or from the ground state (n=1) of the hydrogen atom. The Lyman series is important in astrophysics for studying hydrogen in interstellar space and early universe conditions.

Absorption Spectrum

As we can see from the figure given below that light from a light source falls on a sample of gas. Then it travels through the slit and falls on the prism. So, dispersion of light takes place and then the spectrum can be seen on the detector. The spectrum obtained is the absorption spectrum which is also shown in the figure given below. Such a spectrum will have some dark lines which mean some of the wavelengths have been absorbed by the sample of gas.

When electrons jump to higher energy orbits, they will absorb energy and as there is an absorption of energy, those wavelengths can be seen as dark lines. This is called the absorption spectrum. We can say that the absorption spectrum will have dark lines corresponding to the wavelengths that have been absorbed. So, in simple words, we can say that the absorption spectrum shows the spectrum of Electromagnetic wavelengths passed through a substance, with dark lines due to the absorption of specific wavelengths.

Commonly Asked Questions

Q: What is an absorption spectrum?
A:
An absorption spectrum is a pattern of dark lines or bands that appears when light passes through a gas or other substance. It shows which specific wavelengths of light are absorbed by the atoms or molecules in that substance, revealing information about its composition and structure.
Q: How does an absorption spectrum differ from an emission spectrum?
A:
An absorption spectrum shows dark lines where light is absorbed, while an emission spectrum shows bright lines where light is emitted. Absorption spectra are produced when light passes through a cool gas, while emission spectra are produced when a hot gas emits light.
Q: What causes the dark lines in an absorption spectrum?
A:
The dark lines in an absorption spectrum are caused by atoms or molecules absorbing specific wavelengths of light. When an atom absorbs a photon, its electrons jump to higher energy levels. The absorbed wavelengths correspond to the energy differences between these levels, creating dark lines in the spectrum.
Q: How is an absorption spectrum created in a laboratory?
A:
To create an absorption spectrum in a lab, scientists pass white light through a sample of gas or solution. The sample absorbs certain wavelengths of light, and the remaining light is then passed through a prism or diffraction grating to separate it into its component wavelengths, revealing the absorption pattern.
Q: How does temperature affect absorption spectra?
A:
Temperature can affect absorption spectra by changing the population of atoms in different energy states. Higher temperatures can lead to broader absorption lines due to increased atomic motion (Doppler broadening) and can also affect the intensity of absorption lines.

Applications Of Atomic Absorption Spectroscopy

Following are the applications of atomic absorption spectroscopy.

  • To determine metallic elements in biological materials.

  • To determine calcium, magnesium, potassium and sodium in blood serum.

  • To determine metallic elements in the food industry.

  • It is useful in environmental studies for the detection of metallic impurities in drinking water, soil and ocean.

  • To determine the concentration amount of lead present in petrol, we use atomic absorption spectroscopy.

Commonly Asked Questions

Q: Why are absorption spectra often called "fingerprints" of elements?
A:
Absorption spectra are called "fingerprints" because each element has a unique pattern of absorption lines. This unique pattern allows scientists to identify specific elements in distant stars, planets, and other celestial bodies by analyzing the light that passes through their atmospheres.
Q: What is the relationship between an element's emission and absorption spectra?
A:
An element's emission and absorption spectra are complementary. The wavelengths of light that an element emits when heated are the same wavelengths it will absorb when cool. This relationship is known as Kirchhoff's law of spectral analysis.
Q: How do scientists use absorption spectra to study the composition of stars?
A:
Scientists analyze the light from stars to identify absorption lines characteristic of specific elements. By comparing these lines to known spectra of elements on Earth, they can determine which elements are present in the star's atmosphere and their relative abundances.
Q: What is spectral line broadening?
A:
Spectral line broadening is the widening of absorption or emission lines in a spectrum. It can be caused by various factors, including temperature (Doppler broadening), pressure (pressure broadening), and magnetic fields (Zeeman effect). Broadening provides additional information about the physical conditions of the absorbing or emitting substance.
Q: What is the Beer-Lambert law, and how does it relate to absorption spectra?
A:
The Beer-Lambert law relates the absorption of light to the properties of the material through which it is traveling. It states that the absorbance is directly proportional to the concentration of the absorbing species and the path length of the sample. This law is fundamental in quantitative spectroscopy for determining concentrations of absorbing substances.

Frequently Asked Questions (FAQs)

Q: What is the role of absorption spectroscopy in studying the ozone layer?
A:
Absorption spectroscopy is crucial for monitoring the ozone layer. Ozone strongly absorbs ultraviolet light, and by measuring this absorption from satellites or ground-based instruments, scientists can track the thickness and distribution of the ozone layer. This is vital for understanding and addressing ozone depletion.
Q: How do absorption spectra help in studying the chemistry of comets?
A:
When comets approach the Sun, they release gases that form a coma. By studying the absorption spectra of sunlight passing through this coma, astronomers can determine the composition of the comet's volatile materials. This provides insights into the formation and evolution of the solar system.
Q: What is the concept of hyperfine structure in atomic absorption spectra?
A:
Hyperfine structure refers to small splittings in spectral lines due to the interaction between the electron's magnetic moment and the nuclear magnetic moment. This provides information about nuclear properties and is particularly important in atomic clocks and precision spectroscopy.
Q: How do absorption spectra contribute to our understanding of nuclear processes?
A:
While not directly related to nuclear processes, absorption spectra can provide indirect information. For example, the presence of certain elements or isotopes in stellar spectra can indicate ongoing nuclear reactions in stars. Additionally, studying the spectra of radioactive isotopes helps in understanding nuclear decay processes.
Q: How do absorption spectra help in studying the dynamics of galaxies?
A:
Absorption spectra of galaxies can reveal information about their rotation and internal motions. The Doppler shift of absorption lines indicates the relative motion of different parts of a galaxy, allowing astronomers to study galactic structure, measure rotation curves, and infer the presence of dark matter.
Q: What is the role of absorption spectroscopy in studying planetary atmospheres within our solar system?
A:
Absorption spectroscopy is crucial for studying the composition and dynamics of planetary atmospheres. By analyzing the absorption of sunlight passing through these atmospheres, scientists can identify gases present, measure their abundances, and study atmospheric processes like weather patterns and seasonal changes.
Q: How do absorption spectra change in the presence of magnetic fields (Zeeman effect)?
A:
The Zeeman effect is the splitting of spectral lines in the presence of a magnetic field. This effect can provide information about magnetic field strengths in stars and other astrophysical objects. It's also used in laboratory spectroscopy to study the magnetic properties of atoms and molecules.
Q: What is the significance of the X-ray absorption fine structure (XAFS) in spectroscopy?
A:
XAFS is a technique that uses the fine structure of X-ray absorption spectra to study the local structure around specific atoms in materials. It provides information about bond distances, coordination numbers, and oxidation states, making it valuable in materials science, chemistry, and biology for studying complex structures.
Q: How do absorption spectra contribute to our understanding of stellar atmospheres?
A:
Absorption spectra of stars provide detailed information about the composition, temperature, and pressure of stellar atmospheres. The strength and width of absorption lines can indicate the abundance of elements, while the overall spectral pattern helps determine the star's spectral type and evolutionary stage.
Q: How do absorption spectra help in identifying different isotopes of an element?
A:
Isotopes of an element have slightly different absorption spectra due to their different masses. This causes a small shift in the position of spectral lines, known as the isotope shift. By precisely measuring these shifts, scientists can identify and quantify different isotopes, which is crucial in fields like nuclear physics and geochemistry.
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