Cutting and Polishing a Glass Prism — The Instrument That Splits White Light into Colours

A glass prism is one of the most important instruments in the history of physics. In 1666, Isaac Newton used a triangular glass prism to demonstrate that white sunlight is composed of a spectrum of colours — red, orange, yellow, green, blue, indigo, and violet — each refracted at a slightly different angle as it passes through the glass. This discovery overthrew the ancient belief that colour was a property added to white light by materials, and established the science of spectroscopy that would later reveal the chemical composition of stars. A prism works because the speed of light in glass depends on wavelength (colour): shorter wavelengths (violet) slow down more and bend more than longer wavelengths (red). This dispersion separates the colours spatially. Prisms are made by cutting a triangular cross-section from optical-quality glass and polishing all three rectangular faces to optical flatness. The standard dispersing prism has a 60-degree apex angle. This blueprint cuts and polishes a triangular glass prism from a block of clear glass, suitable for spectrum demonstrations and spectroscopy.