This document discusses the numerical aperture of optical fibers. It begins by introducing optical fibers and their use in transmitting light pulses for information. It then defines the acceptance angle and numerical aperture of a fiber, explaining that NA represents the light gathering capacity and depends on the acceptance angle. The document describes building an experimental setup to measure the NA of a fiber, including assembling wooden planks, laying fiber, and using a laser and photodetector. It presents the Arduino code, intensity vs distance graph, and calculations to determine the fiber's NA of 0.395 based on the experiment's results. Finally, it discusses improvements that could be made to the design and setup.