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1© Manhattan Press (H.K.) Ltd. The composition of electromagnetic waves electromagnetic waves Electromagnetic spectrum Electromagnetic spectrum 8.5 Electromagnetic waves
2 © Manhattan Press (H.K.) Ltd. EM waves & mechanical waves 8.5 Electromagnetic waves (SB p. 17) Mechanical waves: - Require media to propagate Electromagnetic waves: - Do not require media to propagate e.g. water waves & sound waves e.g. light waves & radio waves
3 © Manhattan Press (H.K.) Ltd. EM waves The electromagnetic waves consist of mutually perpendicular time-varying electric and magnetic fields. 8.5 Electromagnetic waves (SB p. 17)
4 © Manhattan Press (H.K.) Ltd. Production of EM waves The energy from the supply is radiated from the plates in the form of a wave which consists of transverse alternating electric and magnetic fields that are mutually perpendicular to each other. 8.5 Electromagnetic waves (SB p. 17)
5 © Manhattan Press (H.K.) Ltd. Both the fields vary sinusoidally and are exactly in phase with each other; and they are at right angles to the direction of propagation. Go to More to Know 6 More to Know 6 Production of EM waves 8.5 Electromagnetic waves (SB p. 18)
6 © Manhattan Press (H.K.) Ltd. Electromagnetic spectrum 8.5 Electromagnetic waves (SB p. 18)
7 © Manhattan Press (H.K.) Ltd. Characteristics of electromagnetic waves 8.5 Electromagnetic waves (SB p. 18) Travel at the same speed of 3 10 8 m s 1 in vacuum Show wave characteristics like reflection, refraction, diffraction and interference Satisfy wave equation: c = f
8 © Manhattan Press (H.K.) Ltd. Production and detection of EM waves 8.5 Electromagnetic waves (SB p. 19) Type of waveWavelengthProductionDetection Radio wave: Long wave Medium wave Short wave = 1 000 m = 100 m = 1 m - 10 m Oscillating charges Resonance electrical circuit Microwave10 2 – 10 3 mOscillating charges Crystal detector, solid-state diode Infrared10 3 – 10 6 m Hot solids, atomic transition, molecular vibration Bolometer, thermopile, skin, photographic film Visible light 4 10 7 – 7 10 7 m Sun, laser, flame Eye, photographic film
9 © Manhattan Press (H.K.) Ltd. Production and detection of EM waves 8.5 Electromagnetic waves (SB p. 19) Type of waveWavelengthProductionDetection Ultraviolet10 7 – 10 9 m Sun, mercury vapour lamp Photocell, photographic film X-ray10 8 – 10 11 m Bombardment of high energy electrons with heavy metal Ionization chamber, Geiger-Muller tube, photographic film Gamma ray 10 11 m Radioactive decay, nuclear fission and fusion reactions
10 © Manhattan Press (H.K.) Ltd. 8.1 Characteristics of waves 1.A pulse carries energy but does not involve the net translation of particles when the wave propagates. 2.The particles of a wave only propagate about their equilibrium positions. 8.5 Electromagnetic waves (SB p. 21)
11 © Manhattan Press (H.K.) Ltd. 8.1 Characteristics of waves 3.For a wave, (a) amplitude is the maximum displacement of a particle from its equilibrium position; (b) wavelength is the distance between two successive particles that are in phase; (c) period is the time required to generate one complete pulse; (d) frequency is the number of completed oscillations per second; and (e) speed is defined as: v = fλ. 8.5 Electromagnetic waves (SB p. 21)
12 © Manhattan Press (H.K.) Ltd. 8.1 Characteristics of waves 4. (a) The intensity I of a wave is the rate at which energy is transferred by the wave to a unit area which is perpendicular to the direction of propagation of the wave. (b) Intensity (Amplitude) 2 5. The displacement-time graph shows the displacement of a particle at different times. 8.5 Electromagnetic waves (SB p. 21)
13 © Manhattan Press (H.K.) Ltd. 8.1 Characteristics of waves 6. The displacement-distance graph shows the displacement of particles at different positions at one instant. 7. The speed of mechanical wave depends on the mass and elasticity of medium. (a) The speed of transverse wave along a stretched string is: where T = tension, = mass per unit length. 8.5 Electromagnetic waves (SB p. 21)
14 © Manhattan Press (H.K.) Ltd. 8.1 Characteristics of waves (b) The speed of longitudinal wave in solid is: where E = Young modulus, = density. 8.5 Electromagnetic waves (SB p. 21)
15 © Manhattan Press (H.K.) Ltd. 8.2 Transverse waves 8. In a transverse wave, the oscillations of particles are perpendicular to the direction of motion of the wave. 8.5 Electromagnetic waves (SB p. 21)
16 © Manhattan Press (H.K.) Ltd. 8.3 Longitudinal waves 9.In a longitudinal wave, the oscillations of particles are parallel to the direction of motion of the wave. 8.5 Electromagnetic waves (SB p. 21)
17 © Manhattan Press (H.K.) Ltd. 8.4 Progressive wave equation 10. For a progressive wave, energy is transferred from the source and propagated outwards. 11. The progressive wave equation is: 8.5 Electromagnetic waves (SB p. 21)
18 © Manhattan Press (H.K.) Ltd. 3.6 Work, energy and power (SB p. 137) 8.5 Electromagnetic waves 12. The electromagnetic waves consist of mutually perpendicular time-varying electric and magnetic fields. 13. All electromagnetic waves travel with the same speed (c) in vacuum: 3 ×10 8 m s –1. 14. The waves in the electromagnetic spectrum are: radio wave, microwave, infrared, visible light, ultraviolet, X-ray and gamma ray.
19 © Manhattan Press (H.K.) Ltd. 8.5 Electromagnetic waves (SB p. 22)
20 © Manhattan Press (H.K.) Ltd. End
21 © Manhattan Press (H.K.) Ltd. Speed of light In 1862, James Clerk Maxwell devised a theory that gives the speed of electromagnetic waves in a vacuum, c, in terms of the electric field constant, ε 0, and the magnetic field constant, μ 0 : The value of ε 0 is 8.85 × 10 –12 F m –1 and that of μ 0 = 4 π × 10 –7 H m –1. When these values are substituted into the above equation, the value of c is 3.0 × 10 8 m s –1, the speed of light which had been determined experimentally. This shows that light waves are electromagnetic waves. 8.5 Electromagnetic waves (SB p. 18) Return to Text
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