Teaching light absorption and the Beer-Lambert law using everyday materials: a tomato juice experiment for introductory physics

TL;DR

The paper addresses how to teach the Beer-Lambert law in resource-limited classrooms by using everyday materials to visualize exponential attenuation of light. It uses tomato juice containing lycopene as the absorber, a halogen lamp as a broadband source, and a compact spectrometer to measure transmission over 200–1000 nm, enabling calculation of absorbance via $A = -log_{10}(I/I_0)$. The results reveal a strong lycopene absorption band near 480–520 nm and show a linear absorbance–concentration relationship at low concentrations with deviations at higher concentrations due to scattering and turbidity. The work provides a safe, portable, and visually engaging demonstration that connects theoretical concepts with real optical measurements, supporting inquiry-based learning and discussions of experimental limitations in optics education.

Abstract

I describe a visually engaging experiment to demonstrate the Beer-Lambert law in introductory physics or general science courses. By using diluted tomato juice as a naturally colored absorber and a halogen lamp as a broadband light source, students can explore how light attenuation depends on concentration and wavelength. The activity connects the optical concept of exponential absorption with everyday materials, making it accessible for classrooms with limited resources. Transmission spectra obtained with a compact spectrometer reveal a strong absorption band around 500 nm, corresponding to the green-blue region absorbed by lycopene. Plotting absorbance against concentration allows students to confirm linear behavior at low concentrations and to discuss deviations at higher ones. The experiment emphasizes conceptual understanding of light-matter interaction, quantitative data analysis, and the limitations of ideal laws, providing a memorable learning experience in optics and spectroscopy.

Figures (4)

• Figure 1: Photograph of the experimental setup used in the classroom activity, showing the halogen lamp (Thorlabs QTH10/M), plastic cuvette (MonotaRO 03004285), compact spectrometer (Thorlabs CCS200/M), and the PC for data acquisition. The simple arrangement makes it easy for students to visualize how light passes through the sample.
• Figure 2: Photograph of the seven cuvettes containing the tomato juice dilution series (0, 2.5, 5, 10, 25, 50, and 100% by volume). The color gradient provides a clear visual representation of concentration differences for students.
• Figure 3: (a) Transmission spectra of 0% (water) and 100% tomato juice, illustrating the pronounced absorption band around 480-520 nm due to lycopene. (b) Absorbance spectra of all seven tomato juice dilutions, plotted as a function of wavelength. The systematic increase in absorbance with concentration demonstrates the applicability of the Beer-Lambert law across the visible range.
• Figure 4: (a) Absorbance versus concentration plots for tomato juice at five wavelengths (500, 600, 700, 800, and 900 nm) over the full concentration range. (b) Expanded view of the dilute region (0-12% v/v), emphasizing the linearity predicted by the Beer-Lambert law.