Explore the fundamental concepts of optical absorption and bandgap in this 28-minute lecture from NPTEL-NOC IITM. Delve into the physics behind how materials interact with light, focusing on the crucial role of bandgap in determining a material's optical properties. Gain insights into the mechanisms of light absorption in semiconductors and insulators, and understand how these principles apply to various technological applications. Learn about the relationship between a material's electronic structure and its ability to absorb or transmit light at different wavelengths. This concise yet comprehensive lecture provides a solid foundation for students and researchers interested in optoelectronics, photovoltaics, and materials science.