This book has been conceptualized as per the recommended National Education Policy (NEP) 2020 and as per syllabus prescribed by Universities of Uttar Pradesh for B. Sc. Students of Physics for the Fourth Semester. This textbook comprehensively covers two papers: Theory and Practical. Part A begins with Structure of Space-Time in Newtonian Mechanics, Galilean Transformation and Electromagnetism Leading to the Foundation of Theory of Relativity is studied in detail. The experimental background of Michelson-Morley Experiment and its Significance of Discarding the Existence of either developed the relativistic kinematics. Inadequacies of Classical Mechanics, Black Body Radiation, Max-Planck's Quantum Hypothesis and Concept of Matter Waves are elaborately explained in a simple manner. Part B deals with the electronics branch which covers transistor biasing, amplifiers, feedback, and oscillator circuits are lucidly explained with suitable examples.