Skip to main content
Biology LibreTexts

2.3: Light spectroscopy

  • Page ID
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)

    Spectrophotometers measure the amount of light absorbed by a sample at a particular wavelength. The absorbance of the sample depends on the electronic structures of the molecules present in the sample. Measurements are usually made at a wavelength that is close to the absorbance maximum for the molecule of interest in the sample.

    The diagram below shows the elements present in a typical spectrophotometer. The light sources used in most spectrophotometers emit either ultraviolet or visible light. Light
    (Io) passes from a source to a monochromator, which can be adjusted to allow only light of a defined wavelength to pass through. The monochromatic (I) light then passes through a cuvette containing the sample to a detector.

    The spectrophotometer compares the fraction of light passing through the monochromator (I0) to the light reaching the detector (I) and computes the transmittance (T) as I/I0. Absorbance (A) is a logarithmic function of the transmittance and is calculated as:

    A = log10(1/T) = log10(I0/I)

    Spectrophotometers can express data as either % transmittance or absorbance. Most investigators prefer to collect absorbance values, because the absorbance of a compound is directly proportional to its concentration. Recall the Lambert-Beer Law, traditionally expressed as:

    A =\(\varepsilon\)b C

    where \(\varepsilon\) is the molar extinction coefficient of a compound, b is the length of the light path through the sample, and C is the molar concentration of the compound. Cuvettes are formulated to have a 1 cm light path, and the molar extinction coefficient is expressed as L/moles-cm. Consequently, absorbance is a unitless value.

    This page titled 2.3: Light spectroscopy is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Clare M. O’Connor.

    • Was this article helpful?