This Arduino-powered spectrophotometer uses a little prism to create rainbows

From arduino.cc

Invented back in the 1940s, a spectrophotometer is a scientific instrument used to measure how different wavelengths of light are absorbed by a sample. Daniel Hingston decided to build his own spectrum analysis device as a fun Arduino experiment, outlined in the project write-up and in the video below.

Inside the 3D-printed device, a servo-driven rotating triangular prism assembly selectively shines parts of the color spectrum through a sample using an E10 filament bulb. Light that passes through the test tube is picked up by an LDR sensor, producing data for different spectral ranges. This info is sent along to a PC over serial, which can then be plotted in a spreadsheet for further analysis.

For complete details, check out the spectrophotometer's project page.