Absorption Spectrum

An absorption spectrum is a powerful scientific tool used to identify the unique wavelengths of light absorbed by atoms, molecules, or materials. In fields like physics, chemistry, astronomy, and spectroscopy, absorption spectra help researchers analyze chemical compositions, study energy transitions, and understand the behavior of matter under different light conditions. From determining the elements in distant stars to enhancing modern optical technologies, absorption spectra play a crucial role in scientific discovery. 

1.0Introduction to Electromagnetic Radiation

• Light is a form of electromagnetic radiation, consisting of oscillating electric and magnetic fields travelling through space. Unlike mechanical waves, electromagnetic waves do not require a medium and can propagate through vacuum with a constant speed:$c=3\times 10^{8}m/s$
• The electromagnetic spectrum spans a wide range of wavelengths and frequencies—from radio waves at the longest wavelengths to gamma rays at the shortest.

Subregions of the electromagnetic spectrum include:

• Microwaves
• Infrared radiation (IR)
• Visible light
• Ultraviolet radiation (UV)
• X-rays
• Gamma rays

Note: Visible white light is only a tiny segment of this huge spectrum.

2.0Definition of  Spectrum

• A spectrum is a visual or graphical display that shows how electromagnetic radiation is distributed across different wavelengths or frequencies. The plural of spectrum is spectra. It helps us understand the composition, behavior, and properties of light or other forms of EM radiation.
• A spectrum represents the range of wavelengths or frequencies present in a source of electromagnetic radiation. It allows scientists to analyze the makeup of light and identify the elements or compounds emitting or absorbing that light.
• When radiation interacts with a medium or is analyzed using optical instruments, it spreads into its component wavelengths. This separation creates a spectrum, revealing how energy is distributed across the electromagnetic range.

Spectra can be observed in daily life, such as:

• A rainbow, which is the natural visible spectrum formed by sunlight refracting in water droplets.
• White light passing through a prism, splitting into multiple colours.
• Diffraction grating, which disperses light into a detailed colour pattern used in spectroscopy.

Visible light splits into the seven colours of the VIBGYOR pattern—Violet, Indigo, Blue, Green, Yellow, Orange, and Red. These colours correspond to specific frequencies:

• Violet: approximately 7.5 × 10¹⁴ Hz
• Red: approximately 4 × 10¹⁴ Hz

Cause of Colour Separation

The splitting of white light occurs because each wavelength bends (refracts) by a different amount when passing through a dispersive medium like a prism. Shorter wavelengths (violet) bend more, while longer wavelengths (red) bend less, resulting in a continuous spread of colours.

3.0Formation of Spectrum

• Spectra are produced when electromagnetic radiation interacts with matter, causing changes in the energy states of atoms or molecules. Depending on how this interaction takes place—whether the matter absorbs, emits, or scatters energy—different types of spectra are formed.

Role of Atomic Structure

• Atoms are made up of electrons that occupy specific, quantised energy levels. These electrons cannot exist between levels; they can only move from one fixed energy state to another. Any such movement requires a precise amount of energy, making electron transitions fundamental to the formation of spectra.

Absorption of Energy

When an atom or molecule receives external energy (from heat, light, or electrical discharge), an electron can absorb a photon and jump to a higher, excited energy level.

• This absorption removes certain wavelengths from the incoming light.
• The resulting pattern appears as dark lines on a continuous spectrum, forming an absorption spectrum.

Emission of Energy

After excitation, electrons cannot remain in the higher energy state for long. They eventually fall back to their lower, stable levels, releasing the absorbed energy as photons of specific wavelengths.

• These emitted photons appear as bright, coloured lines.
• This produces an emission spectrum, characteristic of the element or material.

Types of Spectrum Produced

Depending on whether energy is absorbed or emitted, two major spectra are formed:

• Emission Spectrum: Formed when atoms emit photons as electrons return to lower energy levels.
• Absorption Spectrum: Formed when atoms absorb photons, causing electrons to move to higher energy levels.

Note: Each element has a unique arrangement of energy levels. Therefore, the wavelengths absorbed or emitted are also unique, making spectra powerful tools for identifying substances.

4.0Spectroscopy: Definition and Instruments

• Spectroscopy is the scientific technique used to study and analyze the interaction between electromagnetic radiation and matter. By examining the spectra produced through absorption, emission, or scattering of light, spectroscopy helps scientists determine the composition, structure, and physical properties of different substances—from simple gases to complex molecules.

Key Spectroscopic Instruments

Spectroscopic analysis relies on various instruments designed to observe, record, and measure spectra with accuracy. The most commonly used instruments are:

1. Spectroscope

A spectroscope is a basic optical device that allows direct visual observation of spectra.

• It uses elements such as prisms or diffraction gratings to disperse incoming light into its component wavelengths.
• Spectroscopes are commonly used in educational settings and qualitative analysis to observe emission lines or colour patterns.

2. Spectrograph

A spectrograph records spectra either photographically or digitally. Unlike a spectroscope, which only lets the observer view the spectrum, a spectrograph creates a permanent record.

• Modern spectrographs often use CCD detectors, which provide high sensitivity and resolution.
• They are essential in astronomy, chemical analysis, and remote sensing, where detailed spectral images are required.

3. Spectrometer

A spectrometer is a highly precise instrument designed for quantitative analysis of spectra. It can:

• Measure exact wavelengths of spectral lines
• Identify unknown elements or compounds
• Analyze energy transitions and physical properties such as temperature, velocity, or composition
• Spectrometers are used in laboratories, industries, and research fields requiring accurate numerical data.

Components of a Typical Spectrograph

A spectrograph usually consists of the following key parts:

1. Entrance Slit – Controls the amount of incoming light and improves resolution.
2. Collimating Lens – Makes the light rays parallel before they reach the dispersive element.
3. Dispersive Element (Prism or Diffraction Grating) – Separates light into different wavelengths to form the spectrum.
4. Detector (CCD Sensor or Photographic Plate) – Records the dispersed light to create a visual or digital spectrum.

5.0Classification of Spectrum

Spectrum can be broadly divided into two main categories based on how light interacts with matter:

1. Emission Spectrum
2. Absorption Spectrum

1. Emission Spectra: An emission spectrum is generated when atoms or molecules in an excited state release energy as they return to lower energy levels. The emitted light contains specific wavelengths that correspond to the energy differences between the quantized energy levels of electrons.

Formation of Emission Spectra

1. Atoms or molecules are energized, either by heating, electrical discharge, or other means.
2. The electrons absorb energy and jump to higher energy levels.
3. When the electrons return to their original lower levels, they release energy in the form of photons.
4. The wavelength (or frequency) of each photon corresponds to the energy difference between the two levels, given by: $\Delta E=h\nu$

h is Planck's Constant and is frequency

Types of Emission Spectra

Emission spectra can be further classified into two main types:

A. Continuous Spectrum

• Contains all wavelengths within a certain range without any gaps.
• Produced by dense, hot objects, where atoms and molecules emit light at all wavelengths.
  
  Examples:

1. Rainbow (natural sunlight dispersed by water droplets)
2. Heated solids like the filament of an incandescent bulb
3. Blackbody radiation from hot objects

B. Line Spectrum

• Consists of discrete, sharp lines, each representing a specific wavelength emitted by electrons during allowed transitions.
  Arises from isolated gaseous atoms, where only certain energy transitions are possible.

Example: Hydrogen atomic spectrum (showing Lyman, Balmer, and Paschen series)

6.02. Absorption Spectra

An absorption spectrum occurs when light containing a continuous range of wavelengths passes through a cooler or less energetic medium, such as a gas or vapor. Certain wavelengths are absorbed by the atoms, causing electrons to jump to higher energy levels.

Formation of Absorption Spectra

1. White light (containing all wavelengths) is directed through a cold gas or vapor.
2. Atoms in the gas absorb photons of specific energies, causing electrons to move to higher levels.
3. The transmitted light shows dark lines at the absorbed wavelengths.
4. These dark lines match exactly with the bright lines in the emission spectrum of the same element, making absorption spectra highly specific for chemical identification.

Characteristics of Absorption Spectra

• Consist of dark lines superimposed on a continuous spectrum.
• Each dark line corresponds to a specific energy absorbed by the atoms or molecules.
• Extremely useful for identifying unknown elements or compounds and for astronomical analysis of stars and gases.

7.0Absorption Spectroscopy

• Absorption spectroscopy is a powerful analytical technique used to measure the amount of light absorbed by a sample at different wavelengths. By analyzing how light interacts with matter, this method provides valuable information about the composition, concentration, and structure of substances.

Principle of Absorption Spectroscopy

The technique is based on the fundamental principle that atoms and molecules absorb specific wavelengths of light corresponding to their energy transitions. The process can be summarized as follows:

1. A beam of electromagnetic radiation is passed through the sample.
2. Certain wavelengths are absorbed by the atoms or molecules, while the rest pass through.
3. The reduction in intensity of transmitted light at each wavelength is measured.
4. A graph of absorbance versus wavelength is plotted to obtain the absorption spectrum, which is unique for each substance.

Quantitative Measurement

The amount of light absorbed is expressed as absorbance (A), which is related to the incident and transmitted light intensities:

$A={\log }_{10}\frac{I_0}{l}$

$I_0$= Intensity of incident light, I=Intensity of transmitted light

This relationship is the basis of Beer-Lambert Law, which allows determination of concentration of the absorbing species in a solution.

8.0Comparison Between Absorption and Emission SpectrumChemical analysis, astronomy, detecting unknown substances