ELECTROMAGNETIC WAVES MAXWELL’S EQUATIONS Mawell’s equations: 1. π« π = π π΄π Gauss’s law 2. π« π = π Gauss’s law in magnetism 3. C π = − Δπ« π Δπ Faraday - Neumann law 4. C π = µπ π + ππ π΄π Δπ« π Δπ Ampère-Maxwell law Oscillating charges, that is a variation in the electric field, produce electromagnetic waves ELECTROMAGNETIC WAVES In 1887, Hertz confirmed Maxwell’s prediction when he generated and detected electromagnetic waves Additionally, Hertz showed in a series of experiments that the radiation generated by his device exhibited the wave properties of interference, diffraction, reflection, and refraction, all of which are properties exhibited by light Electromagnetic waves obey the principle of superposition VELOCITY OF ELECTROMAGNETIC WAVES The properties of electromagnetic waves can be deduced from Maxwell’s equations Electromagnetic waves (travelling patterns of electric and magnetic fields) travel through empty space at the speed of light c: π= π π 8 = 3 π₯ 10 πΊπ ππ π THE PROFILE OF ELECTROMAGNETIC WAVES Electromagnetic waves are transverse waves The figure shows that they consist of oscillating electric and magnetic fields that are at right angles to each other and to the direction of wave propagation THE RELATION BETWEEN THE MAGNITUDES OF THE ELECTRIC AND MAGNETIC FIELDS The magnitudes of the electric and magnetic fields in an electromagnetic wave at every instant at any point are related by the expression: πΈ π 8 π = = 3 π₯ 10 π΅ π ENERGY IN ELECTROMAGNETIC WAVES Electromagnetic waves carry energy The energy stored per unit volume in an electromagnetic wave, that is the total energy density is the sum of the energy densities associated with the electric and magnetic fields: 2 2 1 π΅ π΅ π’ = π’πΈ + π’π΅ = π0 πΈ 2 + = π0 πΈ 2 = 2 2π0 π0 Where u, E and B are instantaneous values ENERGY IN ELECTROMAGNETIC WAVES More useful in practice, is the average energy density over time for that, we use the root mean square values of the electric and magnetic fields: 2 π’ = π0 πΈπ ππ where πΈπ ππ = 2 π΅π ππ = π0 πΈππ΄π ; 2 π΅π ππ = π΅ππ΄π 2 ENERGY IN ELECTROMAGNETIC WAVES The rate of energy in a plane electromagnetic wave through a unit area, that is the intensity of the wave I is: πΌ = ππ0 πΈ2 π 2 = π΅ π0 The average energy transported over time is: πΌ= 2 ππ0 πΈπ ππ π 2 = π΅ π0 π ππ The SI unit of the intensity of the wave is W/m2 THE ELECTROMAGNETIC SPECTRUM π = ππ» = π π΄ INTERACTION OF ELECTROMAGNETIC RADIATION AND MATTER When radiation passes through matter it may interact with the material, transferring some or all of its energy to the atoms of that material Ionizing radiation is radiation that transfers energy to the medium by producing ionization of the atoms along its path Non-ionization radiation is referred to any type of electromagnetic radiation that does not carry enough energy to ionize atoms or molecules X-rays or gamma rays, and the higher ultraviolet part of the electromagnetic spectrum are ionizing radiation, whereas the lower ultraviolet part of the electromagnetic spectrum, and also the lower part of the spectrum below UV, including visible light, infrared, microwaves, and radio waves are all considered non-ionizing radiation. POLARIZATION OF ELECTROMAGNETIC WAVES If we consider an electromagnetic wave that travels in the x direction, the figure shows that the electric field π¬ is in the y direction, and the magnetic field π© is in the z direction. Waves such as this one, in which the electric and magnetic fields are restricted to being parallel to a pair of perpendicular axes, are said to be linearly polarized waves. Waves having circular patterns of vibration of the electric and magnetic fields are said to be circularly polarized waves. A light wave that is vibrating in more than one plane is referred to as unpolarized light. Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarised light. It is possible to transform unpolarised light into polarized light by use a polaroid filter.
