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Spectroscopy Lab

Investigate how solutions absorb light using Beer-Lambert Law. Adjust concentration, path length, and molar absorptivity to observe how each factor controls the absorbance and transmittance of light through colored solutions.

Guided Experiment: Beer-Lambert Law Investigation

If you double the concentration of a solution, what do you predict will happen to absorbance and transmittance?

Write your hypothesis in the Lab Report panel, then click Next.

Spectrophotometer

Solution (450 nm absorbed)%T = 1.00%A = 2.0000

Controls

Concentration (c)0.0200 mol/L
Path Length (l)1.0 cm
Molar Absorptivity (ε)100 L/(mol·cm)
Peak Wavelength (λₘₐₓ)450 nm
Band Width (FWHM)50 nm

Results

A=εlc=100×1.0×0.0200=2.0000A = \varepsilon l c = 100 \times 1.0 \times 0.0200 = 2.0000
Absorbance (A)
2.0000
Transmittance
0.010000
%T
1.00%
T=10−A=10−2.0000=0.010000T = 10^{-A} = 10^{-2.0000} = 0.010000
Peak absorption at 450 nm — solution transmits complementary color

Absorption Spectrum

Absorbance vs Concentration

Data Table

(0 rows)
#TrialConcentration(mol/L)Path Length(cm)Absorbance (A)%T(%)
0 / 500
0 / 500
0 / 500

Reference Guide

Beer-Lambert Law

Absorbance increases linearly with concentration and path length.

A=εlcA = \varepsilon l c

Where A is absorbance (dimensionless), ε is molar absorptivity in L/(mol·cm), l is path length in cm, and c is concentration in mol/L. Doubling any single factor doubles absorbance.

Absorption Spectra

Every molecule absorbs light most strongly at its characteristic peak wavelength.

ε(λ)=εmax⁡ e−(λ−λ0)22σ2\varepsilon(\lambda) = \varepsilon_{\max} \, e^{-\frac{(\lambda-\lambda_0)^2}{2\sigma^2}}

The absorption band follows a Gaussian profile. The bandwidth (FWHM) determines how selective the absorption is. Narrow bands indicate highly selective absorbers.

Transmittance

Transmittance is the fraction of light that passes through the solution.

T=10−A%T=T×100T = 10^{-A} \qquad \%T = T \times 100

At A = 0 there is no absorption and T = 1 (100% passes through). At A = 1 only 10% passes through. At A = 2 only 1% passes through. Transmittance drops exponentially as absorbance increases.

Spectrophotometry Applications

Spectrophotometry is a core analytical technique used across science and medicine:

  • Measuring protein and DNA concentration in biochemistry
  • Monitoring reaction progress in chemistry labs
  • Blood glucose and hemoglobin analysis in medicine
  • Water quality and pollutant detection in environmental science
  • Food coloring and quality control in industry

A standard spectrophotometer measures absorbance at hundreds of wavelengths simultaneously to produce a full absorption spectrum.