Lab Procedures, Tips & Safety

Emission Spectrum, Absorption Spectrum & Spectrophotometers

Core Concepts

In this tutorial, you will learn about emission spectrum, absorption spectrum, and what is a spectrophotometer. You will also learn how emission spectra is produced. You will also learn about spectrophotometry and the mechanisms involved, as well as its applications to scientific research. 

Topics Covered In Other Articles

The Structure of an Atom

Discovering the Electron: JJ Thomson and the Cathode Ray Tube

Vocabulary 

  • Photon: particle containing a distinct quantum, or amount, of light energy; basic unit of light
  • Cuvette: a spectrophotometry-specific piece of glassware with straight, clear sides; holds samples during spectrophotometric analyses
  • Prism: transparent, triangular device that bends light, revealing its different wavelengths and their corresponding energies 
  • Spectrophotometer: machine that quantifies the intensity of light transmitted and emitted by a substance, indirectly informing the identification and measurement of the substance 
absorption spectrum and emission spectrum

Introduction to Spectrophotometry and Emission Spectrum

Spectrophotometry informs research in the physical sciences, including chemistry, biology, physics, geology, and astronomy. It facilitates developments in energy, pharmaceutical, health care, agricultural, manufacturing, and forensic realms. 

Spectrophotometry involves the measurement of a substance’s concentration, amount, and identity. This process provides insight into protein synthesis, DNA and RNA function, bacterial cell growth, enzymatic reactions, and more. Below, we cover the mechanisms and applications of spectrophotometry in more detail.

Photons and Waves

Sources of light emit particles called photons. These photons move so fast that, in a single second, each covers 180,000 miles. This flow of photons resembles the shape of a wave; because this electromagnetic wave is composed of discrete parts, or photons, it is not continuous.

The color of traveling light corresponds to the lengths of its photon-comprised waves, which relate to the energies of these photons. Shorter wavelengths denote higher energy levels, while longer wavelengths correspond to lower energy levels. For example, as measured in nanometers (nm), blue and purple light exhibit shorter wavelengths, while red light displays longer wavelengths.

emission spectrum from a spectrophotometer
absorption spectrum from a spectrophotometer

Exploring the Spectrophotometer 

Colors and associated energies are therefore two key components of spectrophotometric investigations. When visible light passes through a prism, its component wavelengths split due to their varying energies and travelling speeds. The spectrophotometer essentially functions as a glorified prism. Scientists place cuvettes of different materials into the machine, which passes light through the samples. When this light contacts the substance, it separates into its various wavelengths

Let’s go over this process and its implications in more detail. Say, for example, scientists want to determine the components of a reaction product. They place this sample into the spectrophotometer, which then floods it with a continuous stream of light particles. The sample may either transmit or absorb each different wavelength of this incoming light; transmitted light passes through the sample, while absorbed light gets taken in by the sample. The spectrophotometer measures the amount of photons that fall into these categories, converting the information into a digital display, or spectrum. Different photons reflect different wavelengths and energies, and thus produce different colors, which appear on these produced spectra. 

How Are Emission Spectra Produced?

Spectrophotometers are responsible for producing both emission and absorption spectra. Light first passes through fiber optic cables and into the spectrophotometer through the entrance slit. This light gets directed onto a grating, dispersing it into its different frequencies, which correspond to energy levels. Concave mirrors ensure that these frequencies then reflect onto a detector. 

Within the detector, photons are converted into digitized electrons. This information feeds into a computer, allowing the software to calibrate and plot it as a function of wavelength and spectral range. This final step produces physical emission and absorption spectra.

Distinguishing Between Emission and Absorption Spectra

Two main types of spectra exist, emission spectra and absorption spectra. Emission spectra involve electrons moving from lower to higher energy levels, which occurs when they take in energy. These excited electrons must then release, or emit, this energy to return to their ground states. The frequencies of this emitted light comprise their emission spectrum. 

Absorption spectra, in contrast, concern light frequencies of electrons that absorb energy. These electrons move from their ground states to higher energy states. The frequencies of this absorbed light comprise their absorption spectrum.

The first diagram above provides an example of an emission spectrum. The colored lines denote discrete transmitted photons carrying discrete amounts of energy. The second diagram above shows an absorption spectrum. Each black line represents a discrete absorbed photon with a discrete energy.

Each element has its own emission spectrum and absorption spectrum, explaining why these displays facilitate the identification and measurement of different substances. In other words, by comparing the spectra produced by their product to literature spectra, the scientists can draw conclusions about the product’s components.

The resolution of the spectrophotometer can change based on the slit width of the machine. Narrowing this slit, which can be accomplished manually, allows the spectrophotometer to account for discrepancies in wavelength, facilitating higher resolution spectra.

History of Line Spectra

Danish physicist Niels Bohr discovered that as electrons orbit an atom, they can travel between energy levels. When an electron jumps from a higher energy level to a lower energy level, it emits photons of light in the process. Conversely, when an electron jumps from a lower energy level to a higher energy level, it absorbs photons of light. 

Bohr studied this process using the hydrogen atom. He saw that distinct colors of light, which corresponded to specific energy quantities, produced discrepancies in the observed “jumping” behavior. He procured spectra describing hydrogen, which showed the colors and affiliated wavelengths absorbed and transmitted by the atom. Bohr’s study of electrons and light spectra intersected with his support of the planetary atomic model and informed the later development of digitized spectra technologies. 

Current Applications of Spectrophotometry 

Experimental analyses often contain a spectrophotometric component, including the production of emission and absorption spectra. But beyond the lab, researchers utilize spectrophotometry for many other investigations. 

Currently, scientists have been attempting to “fine-tune” substances so that they absorb and re-emit light at desired wavelengths. This streamlines the function of solar-powered machines, making them more efficient and effective. Fluorescent materials are also being developed that can absorb and re-emit light at predetermined frequencies, producing new colors. 

Spectrophotometry has also proved useful in clinical contexts. Identification information from different spectra provides insight into the pathophysiology of diseases as well as non-invasive diagnosis efforts.

Additionally, investigators and detectives turn to spectrophotometry to inform their study of criminal evidence. Spectroscopic analytical techniques allow them to discern the elements and molecules, and thus substances, present at crime scenes. 

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