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A Walk Through the Electromagnetic Spectrum

Ken Morgan

ENGR302

May 7, 2002

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The Nature of Light

• Three paradigms…

– Light as an electromagnetic wave

– Light as a stream of particles (photons)

– Light as a probability wave

• Light isn’t just one of these; it’s all three!

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The Nature of Light• Light is an electromagnetic wave

In 1801, Thomas Young showed that light is a wave: Young’s double-slit interference experiment

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The Nature of Light• Light is a stream of particles called photons

The Photoelectric Effects

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The Nature of Light• Light is a probability wave

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The Nature of Light• Maxwell’s Equations

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A time-varying magnetic field is a source of a time-varying electric field.

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The Nature of Light• Maxwell’s Equations

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The sources of a magnetic field are electric currents and time-varying electric fields.

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The Nature of Light• Maxwell’s Equations

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The only source that produces a nonzero flux through a closed surface are free charges.

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The Nature of Light• Maxwell’s Equations

No analog of free charges exists for a magnetic field.

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The Nature of Light• Wave propagation

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This does not exist by itself!

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The Spectrum

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Radio• Wavelengths: ~ 100 km – 1 mm

• Frequencies: ~ 3 kHz – 300 GHz

• Energy: ~ 12 peV – 1.24 meV

Radio from space

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Radio

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Infrared• Wavelengths: ~ 1 mm – 698 nm

• Frequencies: ~ 300 GHz – 430 THz

• Energy: ~ 1.24 meV – 1.78 eV

Anything above absolute zero gives off infrared radiation.

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Visible• Wavelengths: ~ 698 nm – 400 nm

• Frequencies: ~ 430 THz – 750 THz

• Energy: ~ 1.78 eV – 3.1 eV

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Ultraviolet• Wavelengths: ~ 400 nm – 1 nm

• Frequencies: ~ 7.5x1014 Hz – 3x1017 Hz

• Energy: ~ 3.1 eV – 12.4 keV

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X-Rays• Wavelengths: ~ 30 nm – 0.03 pm

• Frequencies: ~ 1016 Hz - 1022 Hz

• Energy: ~ 40 eV – 40 MeV

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Gamma-Rays• Wavelengths: ~ 0.3 nm – 0.3 pm

• Frequencies: ~ 1018 Hz - 1021 Hz

• Energy: ~ 4 keV – 4 MeV

Gamma-ray bursts