What is Optical Spectroscopy?

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 velocity. 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 become visible by emitting or reflecting photons, and the wavelengths of these photons depend on the object's composition, 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 of 620 to 750 nanometers are perceived as red, and so an object that primarily emits or reflects photons in that range looks 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 compose them. The light measured by a spectrometer after the light has been reflected from, passed through, or emitted by the object being studied has what are called spectral lines. These lines are sharp discontinuities of light or darkness in the spectrum that indicate unusually high or unusually low numbers of photons of particular wavelengths. Different substances produce distinctive spectral lines that 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 these as well. In addition to wavelength, other characteristics of the light, such as its intensity, can also provide useful information.

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

One major type of optical spectroscopy, called absorption spectroscopy, is based on identifying which wavelengths of light a substance absorbs by measuring the photons it allows to pass through. The light can be produced specifically for this purpose with equipment such as lamps or lasers or may 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 through. 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 velocity of distant objects by taking advantage of the Doppler effect. Light waves from an object that is moving toward the observer appear to have higher frequencies and thus lower wavelengths than light waves from an object at rest relative to the observer, while the waves from an object that is moving away appear to have lower frequencies. These phenomena are called blueshift and redshift, respectively, because raising the frequency of a wave of visible light moves it toward the blue/violet end of the spectrum, while lowering the frequency moves it toward red.

Another important form of optical spectroscopy is called emissions spectroscopy. When atoms or molecules are excited by an outside energy source such as light or heat, they temporarily increase in energy level before dropping back to their ground state. When the excited particles return to their ground state, they release the excess energy in the form of photons. As is the case with absorption, different substances emit photons of different wavelengths that can then be measured and analyzed. In one common form of this technique, called fluorescence spectroscopy, the subject being analyzed is energized with light, usually ultraviolet light. In atomic emissions spectroscopy, fire, electricity, or plasma is used.

Fluorescence spectroscopy is commonly used in biology and medicine, as it is less damaging to biological materials than other methods and because some organic molecules are naturally fluorescent. 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.