Electromagnetic Radiation

Transcription

1 Electromagnetic Radiation

2 Producing EMR All EMR is produced by accelerating charges Consists of changing electric and magnetic fields Speed of all EMR in vacuum is 3.00 x 10 8 m/s

3 EMR is made up electric and magnetic fields. A changing magnetic field will produce an electric field and vice versa

4 An electromagnetic wave exists when a changing magnetic field causes a changing electric field, which then causes another changing magnetic field, and so on forever. a wave cannot exist unless it is moving. Once created, an electromagnetic wave will continue on forever unless it is absorbed by matter.

5 Transverse wave: E field and B field are mutually perpendicular to direction of travel (see page 643)

6 The electric field of a microwave causes the water molecules to change direction at the same frequency as the EMR Microwave Oven

7 The vibrations causes molecular friction by disrupting hydrogen bonds between neighboring water molecules.

8 Electromagnetic Spectrum

9 Longest wavelength of visible light: ~7.50 x 10-7 m (red light: 750 nm), shortest wavelength 400 nm (violet) see table p 638; pay attention to the method of production and characteristics columns

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12 Saturn seen using infrared, bright regions in south are auroras

13 Transparent material allow light to pass through it easily (you can see through it) Translucent material allows light to pass through, but scatters the light so no image can be seen

14 Wave or Particle? Newton considered light as a stream of particles that obeyed the laws of motion, this explained reflection and refraction of light Newton called the particles of light corpuscles

15 Refraction of light The change in light speed as it travels from 1 substance to another can cause light to bend.

16 Newton s evidence was that: Objects cast sharp shadows; if light was a wave, the shadows would be fuzzy Light passed through a vacuum; there was no material for a wave to propagate through.

17 Huygens proposed that light traveled as a series of waves, which would also explain reflection and refraction as well as diffraction of light but diffraction of light was not easy to measure

18 Diffraction: bending of waves passing through a barrier

19 the wave theory required that light travels as waves in some medium which no one could find (later this medium was called the luminiferous ether)

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21 Maxwell s Equations developed 4 equations that described electric and magnetic field properties the equations predicted the properties of EMR waves

22 produced by the acceleration of electric charges when the charges were moving in simple harmonic motion, the frequency of charge motion would equal the frequency of the EMR predicted the speed of EMR, later verified by experiment predicted all the wave properties: interference, diffraction, refraction, etc.

23 E = ρ ε 0 B = 0 E = B t B = µ 0 + µ 0 ε 0 E t demonstrates that magnetism, electricity and light are different manifestations of the same fundamental laws of electricity and magnetism

24 The EM wave concept remained a theory until demonstrated by Heinrich Hertz in Hertz Experiment Tested Maxwell s equations

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26 The induction coil produced a very high voltage. The field strength between the spheres was strong enough for sparks to jump When the spark jumped, it produced a high frequency electrical oscillation. Hertz Experiment

27 A current was induced in the loop and a small spark jumped across the gap The spark could be made more powerful if the loop was put at the focus of a concave mirror. If the loop was on its side, the effect was not seen at all, indicating that the waves were vibrating in 1 direction. He experimentally verified Maxwell s value for the speed of EMR

28 Wave equation v = λf the wave equation for EMR is usually written as c = λf c = speed of light in vacuum λ = wavelength f = frequency, Hz = s -1

29 the speed of the wave depends on the properties of the medium c = 3.00 x 10 8 m/s IN vacuum Speed is less than c in all other media

30 Remember from Physics 20 that f = 1 T Where T = period of the wave

31 Example Find the frequency of light with a wavelength of 550 nm. f = c λ 550 nm = 550 x 10-9 m f = 3.00 x 10 8 m/s 550 x 10 9 m f = 5.45 x Hz

32 Example Find the wavelength of EMR with a frequency of 800 GHz. λ= c f λ= 3.00 x 10 8 m/s 800 x 10 9 Hz λ= 3.75 x 10-4 m