Why Some of the Rainbow is Missing

In the early 1800s, a German physicist named Joseph von Fraunhofer observed an intriguing phenomenon. While studying sunlight passing through a prism and projecting onto a wall, he noticed that a segment of the rainbow was absent. Across the spectrum from red to violet, there were distinct dark lines where colors should have been. Fraunhofer cataloged over 600 of these missing pieces from the rainbow, varying in intensity. These lines oddly resembled a barcode, serving as a kind of hidden code. Decoding these gaps would later unveil a concealed narrative that would enable scientists to unlock various mysteries of the universe. This is the story of how they achieved this feat and how you can as well.

The scientists who unraveled Fraunhofer's enigma were Gustav Kirchhoff and Robert Bunsen—yes, the same Bunsen of Bunsen burner fame. These two researchers were captivated by the diverse colors emitted by different elements when subjected to a flame. For instance, table salt produces a bright yellow flame, calcium emits an orange hue, and potassium displays a pinkish tinge. To delve into the light emitted more precisely, they devised a tool known as a spectroscope. This instrument directed light from a flame through a prism, dispersing it into its individual wavelengths, creating a spectrum. A secondary tube allowed them to observe the outcome.

When they scrutinized the colorful chemical flames through the spectroscope, they discerned slender bands of light at specific wavelengths. Notably, each element generated a distinct pattern of these bands, akin to a unique fingerprint. Sodium, for instance, exhibited a recognizable yellow line, lithium displayed a vibrant red line, and strontium showcased red lines among others. They even uncovered strong lines for calcium in green and red, alongside numerous fainter lines invisible to the naked eye.

In 1859, Kirchhoff and Bunsen made the groundbreaking revelation that resolved Fraunhofer's puzzle. Acquainted with the missing segments of the rainbow, they sprinkled ordinary table salt onto Bunsen's burner. Upon analyzing the spectral lines emitted by the burning sodium, they realized that these lines coincided precisely with two of Fraunhofer's absent lines. This alignment strongly suggested a connection between the two sets of lines.

By observing both sunlight and a sodium flame simultaneously, Kirchhoff and Bunsen deduced that the bright lines produced by burning sodium correlated with specific dark lines missing from Fraunhofer's rainbow. They postulated that these dark lines resulted from elements in the sun absorbing particular wavelengths of light. This revelation indicated that the spectral lines of individual elements could be matched with the absent lines in the sun's spectrum.

This intriguing breakthrough laid the foundation for deciphering elements present in the sun without requiring direct samples from the distant, blazing celestial body. This method, given the technological limitations of the time, was an insurmountable task. It led to the conclusion that certain elements release light at specific frequencies when heated, yet within a full spectrum of light, they absorb those same frequencies, resulting in spectral lines and gaps. This process occurs when sunlight interacts with elements in the sun's and Earth's atmospheres.

Today, we understand that an element's unique spectral signature is intricately linked to its atomic structure. Atoms consist of nuclei surrounded by electrons orbiting at different energy levels. When atoms absorb energy, such as through heating, electrons transition to higher energy states before swiftly returning to the ground state, emitting photons with precise wavelengths corresponding to the energy absorbed. Elements possess distinct energy level configurations for their electrons, leading to unique emission and absorption spectra.

This profound revelation has profound implications. Spectroscopy now allows us to determine the composition of objects across the universe, from nearby surroundings to galaxies far beyond our reach. It provided hints of the universe's expansion, revealing that galaxies farther away exhibit spectral redshifts, signifying their motion away from Earth. Spectroscopy also guides our search for extraterrestrial life by analyzing starlight filtered through exoplanet atmospheres.

In essence, decoding the hidden messages within sunlight's spectrum unveiled the secrets of our universe's composition, structure, and evolution. It transformed the once-mysterious rainbow into a cosmic encyclopedia, offering insights into celestial phenomena and sparking a quest for knowledge that continues to drive scientific exploration.