What’s Optical Spectroscopy?

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Optical spectroscopy is a means of studying the properties of physical objects based on measuring how an object emits and interacts with light. It can be used to measure attributes such as an object’s chemical composition, temperature, and speed. It involves visible, ultraviolet or infrared light, alone or in combination, and is part of a larger group of spectroscopic techniques called electromagnetic spectroscopy. Optical spectroscopy is an important technique in modern scientific fields such as chemistry and astronomy.

An object becomes visible by emitting or reflecting photons, and the wavelengths of these photons depend on the composition of the object, along with other attributes such as temperature. The human eye perceives the presence and absence of different wavelengths as different colors. For example, photons with a wavelength between 620 and 750 nanometers are perceived as red, so an object that mostly emits or reflects photons in that range appears red. Using a device called a spectrometer, light can be analyzed with much greater precision. This precise measurement, combined with an understanding of the different properties of light that different substances produce, reflect, or absorb under various conditions, is the basis of optical spectroscopy.

Different chemical elements and compounds vary in how they emit or interact with photons due to quantum mechanical differences in the atoms and molecules that make them up. The light measured by a spectrometer after light has been reflected, passed through or emitted by the object under examination has so-called spectral lines. These lines are sharp discontinuities of light or dark in the spectrum that indicate an unusually high or unusually low number of photons of particular wavelengths. Different substances produce distinctive spectral lines which can be used to identify them. These spectral lines are also affected by factors such as the object’s temperature and velocity, so spectroscopy can also be used to measure them. In addition to the wavelength, other characteristics of light, such as its intensity, can also provide useful information.

Optical spectroscopy can be done in several ways, depending on what you are studying. Individual spectrometers are specialized devices that focus on the precise analysis of specific, narrow parts of the electromagnetic spectrum. Therefore they exist in a wide variety of types for different applications.

One of the main types of optical spectroscopy, called absorption spectroscopy, is based on identifying the wavelengths of light a substance absorbs by measuring the photons it passes. The light can be produced specifically for this purpose with equipment such as lamps or lasers, or it can come from a natural source, such as starlight. It is most commonly used with gases, which are diffuse enough to interact with light while still allowing it to pass. Absorption spectroscopy is useful for identifying chemicals and can be used to differentiate elements or compounds in a mixture.

This method is also extremely important in modern astronomy and is often used to study the temperature and chemical composition of celestial objects. Astronomical spectroscopy also measures the speed of distant objects using the Doppler effect. The light waves of an object moving towards the observer appear to have higher frequencies and therefore shorter wavelengths than the light waves of an object stationary relative to the observer, while the waves of an object moving away appear to have frequencies lower. These phenomena are called blueshift and redshift, respectively, because raising the frequency of a visible light wave shifts it towards the blue/violet end of the spectrum, while lowering the frequency shifts it towards the red.
Another important form of optical spectroscopy is called emission spectroscopy. When atoms or molecules are excited by an external energy source such as light or heat, they temporarily increase their energy level before returning to their ground state. When the excited particles return to their ground state, they release excess energy in the form of photons. As with absorption, different substances emit photons of different wavelengths which can then be measured and analysed. In a common form of this technique, called fluorescence spectroscopy, the test subject is energized with light, usually ultraviolet light. In atomic emission spectroscopy, fire, electricity or plasma are used.

Fluorescence spectroscopy is commonly used in biology and medicine because it is less harmful to biological materials than other methods and because some organic molecules naturally fluoresce. Atomic absorption spectroscopy is used in chemical analysis and is particularly effective for detecting metals. Different types of atomic absorption spectroscopy are used for purposes such as identifying valuable minerals in ores, analyzing evidence from crime scenes, and maintaining quality control in metallurgy and industry.

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