Summary: The text discusses various concepts in physics, including optics, photometry, and spectroscopy. In optics, the Michelson experiment is used to measure the velocity of light, while comatic aberration and astigmatism are discussed as defects in images formed by lenses. Reflection at plane and curved surfaces is also explored, including the concept of a sextant used to measure angular distances between celestial objects. The article also covers photometry, luminous efficiency, and the principles of optics, including the behavior of convex and concave lenses and the concept of the optical centre. Additionally, the text discusses the application of lenses in optical instruments, including the calculation of focal length and the use of microscopes. Finally, the importance of spectroscopy in science and industry is highlighted, with its applications in fields such as medicine, astronomy, and materials science.