Quantitative measurement of birefringence in transparent films across the visible spectrum

TL;DR

This study presents an affordable method for quantitatively measuring birefringence in household polymer films across the visible spectrum, using incoherent light and spectrometry, suitable for educational settings.

Contribution

It introduces a novel, cost-effective experimental setup that combines incoherent light sources and spectrometry for birefringence measurement in common films.

Findings

Measured birefringence spectra of six household films.

Demonstrated use of interference effects to determine film thickness.

Validated the method as an educational tool for optics experiments.

Abstract

Common transparent polymer films such as cellophane and household tape are frequently used as examples of birefringent materials in textbooks and classroom demonstrations. Qualitatively, birefringence is often demonstrated by layering such films between crossed-polarizers. In this work, we describe an inexpensive experimental setup for the quantitative measurement of birefringence in common household films, suitable for senior high school or undergraduate labs. Whereas traditional approaches for polarization-based classroom experiments typically use monochromatic laser light, we encourage the combined use of an incoherent incandescent light source and portable spectrometer. In addition, we demonstrate how any concomitant thin-film interference effects can be used to separately measure the optical thickness in the most heterogeneous and uniform films. Such measurement can then be used as an independent experimental confirmation of either the film's index of refraction or its thickness, given knowledge of the other. In an effort to provide examples for the data analysis procedures as well as to investigate a range of materials, we measure the birefringence across the visible spectrum of six common household polymer films, including thin kitchen wrap, cellophane, gift basket film, and common adhesive tapes.