Electromagnetic WavesMind Map

Displacement current was introduced by Maxwell to remove an inconsistency in Ampere’s circuital law. During charging of a capacitor, conduction current flows in the wires, but no actual charge crosses the gap between plates. Still, the magnetic field around the circuit is continuous, so Maxwell proposed that a changing electric field between the capacitor plates acts like a current. This is called displacement current, ID = ε0 dΦE/dt. It is not due to physical flow of charges across the dielectric or vacuum gap, but due to changing electric flux. Including it gives Ampere-Maxwell law: ∮B·dl = μ0(IC + ID). Displacement current is essential for continuity of current and for the existence of electromagnetic waves.

Maxwell predicted that changing electric and magnetic fields can sustain each other and travel through space as electromagnetic waves. An electromagnetic wave consists of oscillating electric field and magnetic field that are perpendicular to each other and also perpendicular to the direction of propagation. Hence EM waves are transverse waves. They do not require any material medium and can travel through vacuum. In vacuum their speed is c = 1/√(μ0ε0), equal to approximately 3 × 10⁸ m/s. The electric and magnetic fields are in the same phase, and their amplitudes satisfy E0/B0 = c. All electromagnetic waves obey v = fλ, where f is frequency, λ is wavelength and v is wave speed.

The electromagnetic spectrum is the continuous arrangement of electromagnetic waves according to wavelength or frequency. All members are electromagnetic waves and travel with the same speed in vacuum, but they differ in wavelength, frequency, energy, production and applications. In decreasing wavelength order, the spectrum is radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays and gamma rays. Frequency and photon energy increase in the opposite direction. Radio waves are used in broadcasting and communication, microwaves in radar and ovens, infrared in thermal imaging, visible light in vision, ultraviolet in sterilization, X-rays in medical imaging and gamma rays in cancer treatment. NEET frequently asks the correct order and applications.

Electromagnetic waves are used in daily life, medicine, communication, industry and research according to their wavelength, frequency and penetration ability. Radio waves are used in communication systems such as broadcasting, mobile communication and satellite links. Microwaves are used in radar because they reflect from aircraft and ships, and in microwave ovens because water molecules absorb microwave energy and heat food. Infrared radiation is used in remote controls, thermal imaging and night vision. Ultraviolet rays kill microorganisms and are used for sterilization. X-rays penetrate soft tissues but are absorbed more by bones, making them useful in medical imaging. Gamma rays have very high energy and are used in cancer treatment to destroy malignant cells.