Revisiting the round bottom flask rainbow experiment

TL;DR

This paper analyzes how the vessel's wall in the round-bottom flask rainbow experiment affects the formation and appearance of optical bows and Alexander's dark band, providing insights for accurate demonstration setup.

Contribution

It reveals the impact of the flask's wall on bow positions and dark band formation, offering guidance for improved experimental accuracy.

Findings

Wall shifts the first and second-order bows closer together

Wall causes splitting of the bows, potentially misleading interpretation

Proper distance between flask and screen is crucial for clear Alexander's dark band

Abstract

A popular demonstration experiment in optics uses a round-bottom flask filled with water to project a circular rainbow on a screen with a hole through which the flask is illuminated. We show how the vessel's wall shifts the second-order and first-order bows towards each other and consequentially narrows down Alexander's dark band. We address the challenge this introduces in producing Alexander's dark band, and explain the importance of a sufficient distance of the flask to the screen. The wall-effect also introduces a splitting of the bows which can easily be misinterpreted.