This textbook has been designed to meet the needs of B.Sc. Third Semester students of Physics as per Common Minimum Syllabus prescribed for all Uttar Pradesh State Universities and Colleges under the recommended National Education Policy 2020. Maintaining the traditional approach to the subject, this textbook comprehensively covers both the parts of the theory papers, namely, Electromagnetic Theory and Modern Optics as well as the Practical Paper. The theory part includes important theoretical topics such as Electrostatics, Magnetostatics, Time Varying Electromagnetic Fields, Electromagnetic Waves, Interference, Diffraction, Polarisation and Lasers are aptly discussed to give a complete overview of Electromagnetic Theory & Modern Optics. The practical part covers experiments which are on Carey Foster bridge, Earth inductor, deflection and vibration magnetometer, study of variation of magnetic field along the axis of a single and double coil. Ballistic galvanometer-based experiments to determine high resistance, low resistance, self-inductance and comparison of capacitances are explained in detail.