Explore the fascinating world of optics in this advanced undergraduate lecture from the "Light and Beyond" summer course. Delve into the importance of optics in science and technology, examining practical applications like X-ray crystallography and interferometric imaging. Discover the theoretical foundations of geometrical optics, including Fermat's principle and the law of refraction. Learn about historical contributions from figures like Heron of Alexandria and engage with concepts such as least path reflection and stationarity. Gain insights into how the principle of least time explains refraction, and participate in a Q&A session to deepen your understanding of these fundamental optical phenomena.