Short Answer

An absorption spectrum is the pattern of dark lines or gaps that appear when a continuous beam of light passes through a gas or substance, and certain wavelengths of light are absorbed by the atoms. These dark lines show the wavelengths that the atoms have absorbed to move their electrons to higher energy levels.

Each element produces its own unique absorption spectrum because every atom absorbs only specific wavelengths. Absorption spectra help identify elements in stars, gases, and unknown materials.

Detailed Explanation :

Absorption spectrum

An absorption spectrum is a type of atomic spectrum formed when atoms or molecules absorb specific wavelengths of light. When white light or continuous radiation passes through a cool gas or substance, the atoms in the gas absorb certain wavelengths to excite their electrons from lower energy levels to higher levels. As a result, these wavelengths are missing from the transmitted light and appear as dark lines when viewed through a spectroscope. These dark lines collectively form the absorption spectrum.

The absorption spectrum provides important information about the internal structure of atoms and the arrangement of their energy levels. This is because each element absorbs light at very specific wavelengths, corresponding to the exact energy differences between electron levels. Therefore, the absorption spectrum is considered the fingerprint of an element. It helps scientists identify unknown substances, study stars, and understand atomic behaviour.

Absorption spectra support the concept of quantized energy levels. Electrons can move to higher energy levels only by absorbing a fixed amount of energy. If the energy (or wavelength) of incoming light matches the required energy difference between levels, absorption occurs and a dark line appears in the spectrum.

Formation of absorption spectrum

The formation of an absorption spectrum involves several steps:

Source of continuous spectrum
A hot object like the sun or a filament lamp produces continuous light that contains all wavelengths.
Passage through a cool gas
As this continuous light passes through a cooler gas, the atoms in the gas absorb specific wavelengths that match the energy gaps between their electron levels.
Excitation of electrons
When an atom absorbs light, one of its electrons jumps from a lower energy level to a higher one. This requires a precise amount of energy.
Dark lines in the spectrum
The absorbed wavelengths are missing in the transmitted light. When this light is viewed through a spectroscope, missing wavelengths appear as dark lines, forming the absorption spectrum.

These dark lines directly correspond to the bright lines seen in the emission spectrum of the same element. This is because the energy absorbed during excitation is the same energy released during emission.

Types of absorption spectra

Absorption spectra can be of different types, depending on the nature of the absorbing substance:

Atomic absorption spectrum
This shows sharp dark lines at specific wavelengths. It is produced when free atoms in a gas absorb light.
Molecular absorption spectrum
This shows bands instead of sharp lines, because molecules have more complicated energy levels.
Continuous absorption spectrum
Some solids and liquids absorb broad ranges of wavelengths, producing a general decrease in intensity instead of sharp lines.

Characteristics of absorption spectrum

The absorption spectrum has several important characteristics:

Dark lines: It contains dark lines where light has been absorbed.
Uniqueness: Each element produces its own unique pattern.
Relation to energy levels: Each line represents an electron transition from a lower level to a higher level.
Exact match with emission spectrum: The positions of dark lines match the bright lines in the emission spectrum.
Evidence for quantization: The spectrum proves that electrons can absorb only certain fixed energy amounts.

These features make the absorption spectrum a powerful tool in scientific study.

Applications of absorption spectrum

The absorption spectrum has many important applications in physics, chemistry, astronomy, and industry.

Identifying elements in stars
By studying absorption spectra of sunlight and starlight, scientists can find out which elements are present in stars. For example, helium was first discovered in the sun by analysing its absorption spectrum.
Understanding atomic structure
Absorption spectra give information about the energy levels in atoms. The pattern of lines helps scientists understand the arrangement and spacing of levels.
Chemical analysis
Spectroscopic instruments use absorption spectra to identify elements in liquids, gases, soil, and biological samples.
Environmental and industrial testing
Industries use absorption spectroscopy to check pollution levels, metal purity, and chemical composition.
Astronomy and space research
Absorption spectra help determine the temperature, composition, and motion of distant stars and galaxies. Shifts in absorption lines reveal whether a star is moving toward or away from Earth.
Medical and biological fields
Absorption spectra help analyse blood samples, detect chemicals, and study biological molecules.

Relation to quantum mechanics

Absorption spectra played a major role in the development of quantum mechanics. Classical physics could not explain why atoms absorb only specific wavelengths. Bohr’s atomic model explained it by introducing quantized energy levels. Later, quantum mechanics provided a deeper understanding by describing electrons through wave functions and orbitals.

Absorption spectra remain one of the strongest proofs of the quantum nature of atoms and electrons.

Conclusion

An absorption spectrum is a pattern of dark lines formed when atoms absorb specific wavelengths of light to excite their electrons. Each element has its own unique absorption spectrum because its energy levels are unique. Absorption spectra help identify elements, study atomic structure, analyse stars, and support quantum theory. They are widely used in science, industry, and astronomy.