Understanding the Absorption Spectrum
An absorption spectrum is a unique pattern of dark lines or bands that appears when electromagnetic radiation (like light) passes through a substance, and specific wavelengths are absorbed. These absorbed wavelengths correspond to energy levels within the atoms or molecules of the substance, causing them to transition from a lower energy state to a higher, excited state.
Key Principles of Light Absorption
The absorption process occurs because atoms and molecules can only absorb photons with energies that exactly match the difference between two of their quantized energy levels. These energy levels relate to electron transitions, molecular vibrations, and rotations. When a photon's energy matches one of these differences, it is absorbed, leaving a 'gap' or dark band in the continuous spectrum of light that initially passed through.
A Practical Example: Stellar Spectroscopy
A classic example is the Sun's absorption spectrum. When sunlight is passed through a prism, a rainbow of colors (a continuous spectrum) is observed, but it is crossed by many dark lines, known as Fraunhofer lines. These lines indicate specific wavelengths absorbed by elements like hydrogen, helium, and sodium in the Sun's cooler outer atmosphere, revealing its chemical composition.
Importance and Applications
Absorption spectra are crucial tools across various scientific disciplines. In chemistry, they act as 'fingerprints' for identifying unknown compounds and determining concentrations. In astronomy, they help determine the composition of stars, planets, and interstellar gases. In biology, they are used to study pigments like chlorophyll and hemoglobin, understanding how they absorb light.