absorption spectra
Summary
An absorption spectrum shows which wavelengths a substance absorbs. Because energy levels are quantized, an atom absorbs photons whose energy matches the gap between two levels, leaving dark lines. Those lines fall at the same wavelengths as the element's emission lines.
Detail
Meaning: An absorption spectrum shows which wavelengths a substance absorbs. Atomic energy levels are quantized, so a photon is absorbed in a transition between bound levels only if its energy equals the gap between two levels. White light passing through a cool gas therefore emerges with dark lines on a bright background, a pattern characteristic of each element.
Equation: ΔE = Ephoton = hf = hc/λ, where ΔE is the gap between levels (J), h is Planck's constant (6.63 × 10⁻³⁴ J·s), f is frequency (Hz), c is the speed of light (3.00 × 10⁸ m/s), and λ is wavelength (m).
On the MCAT: Given an energy-level diagram, you identify which transition matches a dark line: a larger gap means a higher frequency and a shorter wavelength. Passages may also show a UV-visible spectrum, where peak position helps identify the compound and peak height rises with concentration.
Common confusion: Students think absorption and emission lines for the same element differ in position. They sit at the same wavelengths, because both reflect the same level gaps. Only the appearance differs (dark on bright versus bright on dark). Also, a solution that looks blue is absorbing other colors, such as orange light, and transmitting blue.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- Physics (Cutnell and Johnson)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.