Introductory Astronomy Complete University Course - The Historical Understanding of Light

This video is part of a comprehensive series initially developed for William Paterson University and CUNY Hunter, aimed at supporting online classes and course materials for introductory astronomy. By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! Last time, we discussed the nature of light governed by wave principles. Now, let’s explore the historical journey of understanding light and its properties. The journey began with Euclid around 300 BC, who posited that light reflects off surfaces, with the angle of incidence equaling the angle of reflection, suggesting light behaves like a particle. Claudius Ptolemy, around 130 AD, discovered that light refracts when moving from one medium to another, though he couldn’t establish the precise mathematical relationship. Nearly 800 years later, Iraqi philosopher Ibn Sahl (c. 980 AD) derived the law of refraction, showing the ratio of the sine of the angle of incidence to the sine of the angle of refraction is proportional to the speed of light in different media. Around 1020 AD, Ibn al-Haytham revolutionized the concept of vision by demonstrating that light is received by the eye rather than emitted from it. Using a camera obscura, he showed how light entering a small hole creates an inverted image, analogous to how the eye functions. In 1307, Theodoric of Freiburg in Germany studied rainbows by using water-filled glasses to demonstrate how light refracts and reflects within water droplets, forming rainbows. By 1660, Francesco Grimaldi demonstrated the wave nature of light through diffraction experiments, showing that light spreads out after passing through a slit, similar to water waves. These discoveries set the stage for Isaac Newton.