top of page
Search
Writer's pictureEngineering Pivot
Aug 21, 20246 min read

Spectroscopy: Our eyes into the universe

By Saatvik Sanjay 




By David Clode via Unsplash.com



For the longest time, spectroscopy has gone unnoticed for its contributions to our modern understanding of physics. So today, I’m going to give it the recognition it deserves and tell you all about how spectroscopy shaped the way we view our universe today. 


What is Spectroscopy? 

Spectroscopy is the study of emission and absorption of light and other radiations by types of matter. The word “spectroscopy” is derived from the Latin word “spectrum” meaning light and the Greek word “skopia” which means observation. The art of spectroscopy all began with Newton and his analysis of light. Newton had already proved that the sun’s white light could be split into several colors and coined the term spectrum for this. The earliest spectroscope was invented by him and consisted of a small aperture that allowed the light to enter, a lens to make the rays parallel to one another, a glass prism to split or disperse the light, and a screen to record the resulting spectrum. In fact, this is pretty similar to the spectroscopes we get today. 


Much of the quantitative value of spectroscopy however came from German physicist Joseph von Fraunhofer. He collected spectra from many planets and stars which laid down the foundation for astrophysics. 


He also invented the process of diffraction grating. The gratings that are used work more or less on the same principle of your typical prism. Except, this time the light isn’t separated based on wavelength, but instead because of periodically spaced grooves on its surface. Through the process of diffraction, they split the light into different wavelengths (or colors) and cause them to propagate in different directions. In contrast to prisms, however, the lower wavelengths are closer to the incident rays (the rays of light falling on the surface). So in this case, an inverted spectrum of colors will be formed, with violet being the shortest on top and red being the longest on the bottom.


What do we use Spectroscopy for? 

Okay, so spectroscopy helps us to make boring old white light into colored light. So what? What good does that do? 


Well, to put it into perspective – spectroscopy has played a crucial role in developing quantum mechanics, quantum electrodynamics, and even Einstein’s theory of special relativity! It has also helped us understand the electromagnetic force and the fundamental strong and weak forces. Whether we see it or not (see what I did there?), spectroscopy has been a vital addition to modern physics. 


Spectroscopy isn’t unique to any one field, in fact, it is used in a variety of disciplines. It is most commonly used in astronomy to distinguish celestial bodies based on the spectra and wavelength of the light. However, it is not limited to that, because some form of spectroscopy is used in chemistry and particle physics. It allows us to understand the physical and chemical makeup of an object.


 Apart from that, it is used in MRIs (magnetic resonance imaging) and X-rays and is even used to test doping in sports. 


Types of Spectra

As discussed before, we use the process of diffraction to split the light into its component colors and onto a spectrum. Even though we can display the spectra as an image, it is usually plotted as a graph to avoid missing any subtle differences that are too faint for our eyes to detect. These spectra help us identify the element the light has passed through or been absorbed by, as each element has its unique signature on the spectra.


There are mainly three types of spectra that are formed: continuous, emission, and absorption spectrum. At their most basic level, all spectra show the same thing which is how brightness varies with wavelength. They are classified based on how they display light and matter interactions and how they can be used. 


  • Continuous Spectrum: In a continuous spectrum, there are little to no missing colors and the brightness varies evenly from one color to another. A prime example of this is the blackbody curve. The blackbody curve is the band of colors that an object emits based on its surface temperature. These are useful because the shape of the curve and the peak wavelengths are directly related to how hot the surface is and nothing else. So, it is ideal for helping us figure out the temperature of an object. For example, hotter and brighter stars emit more blue light than red while cooler and dimmer stars emit more red light than blue. 


  • Absorption Spectrum: This spectrum is similar to a continuous spectrum except it has colors that are completely missing. These colors can be seen as dark lines (absorption lines) which are, as you might expect, due to the absorption of light. When starlight passes dense materials such as gas, atoms absorb certain wavelengths. The interesting part and perhaps most beneficial part of this is the fact that each element only absorbs specific types of wavelengths (we’ll get to this in a moment). So, if we recognize this signature, we can identify the element that exists. The relative darkness of each absorption line also helps us understand the different amounts of materials along with the temperature and density of the gases it passes through. 


  • Emission Spectrum: The emission spectrum is the opposite of an absorption spectrum – it is mostly dark with colored lines or emission lines. These lines are also characteristic of specific atoms and in fact, the wavelengths of an emission line of an atom are exactly the same as the wavelengths of its absorption lines.


The German physicist Gustav Kirchoff summarized how to create these different types of spectra in his three laws of spectroscopy: 

  1. A luminous solid, liquid, or dense gas emits light of all wavelengths.

  2. A low-density, hot gas seen against a cooler background emits an emission spectrum.

  3. A low-density, cool gas in front of a hotter source of a continuous spectrum creates an absorption spectrum.

To better understand these spectra and Kirchoff’s laws, we have to go down to meet our humble friend, the electron. Let’s shed light on spectroscopy and how it works the way it does (pun intended). 


How does Spectroscopy work? 

The basic principle behind spectroscopy is that different materials emit and interact differently with different wavelengths of light based on factors such as temperature and composition. The spectroscopy techniques are also extremely sensitive as single atoms and even different isotopes (basically the same element but with a different atomic mass resulting in different characteristics) can be detected. This is all thanks to the electron and the way it exists in the first place. 


Without going too much into atomic physics, let’s start with how an electron exists in an atom. First proposed by Niels Bohr, the simplified explanation is that electrons occupy specific orbits around the nucleus and can only exist in these orbits, not in between. Each orbit also has a discrete amount of energy associated with it – no more, no less – so it can also be called energy levels. An electron has the ability to jump up and down these energy levels based on the amount of energy it absorbs or emits. It also takes an exact amount of energy to make it move up or down energy levels. This energy that it requires corresponds to the specific wavelength of light that it absorbs/emits. 


To understand this more clearly, let's take a look at a hydrogen atom, the simplest atom in the universe. Based on the absorption spectrum of hydrogen, it absorbs light of only 4 different wavelengths – 410 nm, 434 nm, 486 nm, and 656 nm (nm stands for nanometers). Let’s consider the shortest wavelength, 410 nm. 


The shortest wavelength has the highest energy. So, when violet light with exactly 410 nm comes in contact with hydrogen, it gets absorbed and causes the electrons to jump up four energy levels. However, the inverse is also true. When the electron loses energy and falls four energy levels, it emits photons with the same wavelength, that of 410 nm. This is why the emission spectrum is the exact inverse of the absorption spectrum. 


The best part of all of this is that each element absorbs or emits unique wavelengths based on its electrons. Thus, we can use these spectrums to identify precisely what element it is. 


Conclusion

While spectroscopy may seem a little far-fetched for your average Joe, the truth is we use it every day without even knowing it. Our ability to see color and make predictions about things based on their color is based on spectroscopy. For instance, we can tell the difference between skimmed milk and whole milk simply by looking at it, because whole milk looks “thicker” than skimmed milk. Similarly, we can also tell the difference between incandescent, neon, or fluorescent light because of how the light shines even if they’re all the same color.

In the end, it's amazing to know that a few colors can tell us so much we don’t know about our universe. Hope you enjoyed! 


Works cited



bottom of page