Frameworked.
Back to the workbench

A SMALL EXPERIMENT / Spectroscopy

Light through a solution.

Change concentration and path length to see how absorbance and transmitted light are related.

03 / LIGHT & CONCENTRATIONIdeal solution

Follow the light.

Light transmission through a sample and absorbance versus concentrationPlots load when JavaScript is enabled.

Beam opacity represents relative transmitted intensity, not the solution’s actual color. The calibration line uses the current path length.

Change the sample.

A longer path gives the same solution more opportunity to absorb light.

The relationship

Absorbance grows linearly with concentration and path length. Transmission decreases exponentially with their product.

Absorbance, A
—
Light transmitted
—
Light absorbed
—

What changes

Absorbance is linear in concentration; transmission is exponential.

The model, explained.

A=εℓc=−log⁡10TA = \varepsilon\ell c = -\log_{10} T
T=II0=10−AT = \frac{I}{I_0} = 10^{-A}

We hold the molar absorption coefficient ε at an illustrative 10,000 L mol⁻¹ cm⁻¹ at one fixed wavelength. ℓ is path length in cm, and c is concentration in mol/L. The slider uses µM, converted to mol/L before calculation. Absorbance has no units.

This assumes a homogeneous, dilute solution with unchanged chemical speciation and monochromatic light. Reflection, scattering, stray light, and detector limits are excluded, so light absorbed is 100% minus light transmitted. The graph is an ideal calibration, not experimental data.

Read about transmission spectroscopy from the Coblentz Society