A bright line spectrum, also known as an emission spectrum, is a pattern of distinct, colored lines separated by dark spaces. This discontinuous display of light is produced when atoms of a specific element release energy. Unlike the continuous rainbow of light from a traditional incandescent bulb, the line spectrum reveals only a few specific wavelengths. This unique pattern serves as an “atomic fingerprint,” allowing scientists to identify elements by analyzing the light they emit.
Quantized Energy Levels in Atoms
The reason a spectrum appears as discrete lines rather than a continuous band of color lies in the structure of the atom. Electrons are restricted to specific, fixed shells or energy levels, rather than orbiting arbitrarily. This concept is known as energy quantization, meaning energy can only exist in certain discrete amounts, much like climbing a staircase where you can only stand on the rungs, not in the space between them.
Because electrons are confined to these specific levels, they can only gain or lose energy amounts that exactly match the difference between two fixed levels. This contrasts with the classical idea of continuous energy, which would produce a smooth, unbroken spectrum of light. The fixed nature of these allowed energy states is the prerequisite for the formation of a line spectrum.
The Excitation of Electrons
To produce the light of the bright line spectrum, atoms must first be energized, a process called excitation. This typically involves subjecting the atoms, often gaseous, to an external energy source like heat or an electrical discharge. For example, high voltage electricity is passed through the gas inside a neon sign tube.
The energy absorbed by the atom is transferred to its electrons, forcing them to jump from their stable, lower-energy orbits (the ground state) to higher, less stable orbits. The atom is now in an excited state. This newly acquired energy must be released for the atom to return to its stable, ground state configuration.
Photon Emission and Discrete Wavelengths
The relaxation process is the mechanism that creates the light of the spectrum. An excited electron immediately falls back down to a lower, more stable energy level. When the electron moves from a higher energy state to a lower one, the excess energy it initially absorbed is released as a tiny, discrete packet of light energy, called a photon.
The energy of the emitted photon is precisely equal to the difference in energy between the two electron levels involved in the drop. Since energy levels are fixed, the amount of energy released is also fixed and discrete, not continuous. This specific, fixed amount of energy corresponds directly to a single, specific wavelength (color) of light, which is why the spectrum shows individual bright lines.
Multiple lines appear in the spectrum because the electron can return to its ground state through various possible transitions between different energy levels. For instance, a drop from the third level to the first level releases a different, larger amount of energy than a drop from the second level to the first level. Each unique transition corresponds to a different fixed energy value, which produces a photon with a distinct color and wavelength, forming the complete bright line spectrum for that element.
Analyzing the Spectral Fingerprint
Once the light is emitted from the energized atoms, a spectroscope or spectrograph is used for analysis. This device uses a prism or a diffraction grating to separate the mixed light into its individual component wavelengths. The spectroscope spreads out the light, making the discrete lines of color visible against a dark background.
The resulting pattern of bright lines is unique for every element, allowing for definitive identification. Scientists compare the observed pattern of lines and their specific wavelengths to known reference spectra to determine the element’s identity. This technique is powerful in astronomy, where spectrographs attached to telescopes analyze the light from distant stars and galaxies to determine their chemical composition.