# A Resonance Applet fendt.de/ph11e/resonance.htm.

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A Resonance Applet http://www.walter- fendt.de/ph11e/resonance.htm

A quick review Simple oscillations period, frequency, amplitude Many simple oscillations -> waves Wavelength, Speed = wavelength X frequency Electromagnetic waves are produced by oscillations of charges!

Electromagnetic Waves Charges and Electric Fields Charge Oscillation and Generation of Electromagnetic waves Speed, Frequency and Polarizations Spectrum of Electromagnetic Radiation What Is Light?

Electric Charges and Fields There are two kinds of electric charges: Positive and negative. The same charges repel and the different ones attract. There are electric fields (lines) around charges. Charges interact with the fields + --

When the charges oscillate, so do the the electric field lines which send out ripples. The ripples can be created in the directions orthogonal to the direction of oscillation (transverse wave) When a positive charge oscillates against a negative one, the ripples are loops of electric fields which propagate away from the charges. Electromagnetic waves

Oscillating charges also generate a oscillating current, which in turn produces oscillating magnetic field ripples propagating away. The propagating electric and magnetic ripples interact with and support each other. They can go forever until absorbed by some charges along the way. They are electromagnetic waves!

Summary of Concepts Electric and magnetic fields of Faraday can go off the wiggling charges, and propagate in space by themselves! (Maxwell, 1861-1862) Hertz confirmed this by experiments in 1888. Electromagnetic wave is a transverse wave. The E&M wave waves without a medium!

E&M Radiation: Frequency and wavelength C=  ( Speed is a constant in vacuum) Radio wave: = a few meters, ~ 1 –10 MHz 1 M (mega) = 10 6 ( oscillation freq. in a circuit) Microwave: = centimeters, ~ 1 –10 GHz 1 G (giga) = 10 9

Light   = 300-1500 n(ano)m,  ~ 10 14 –10 15 Hz  (Resonance frequency of the receptors in eyes) 1 nanometer = 10 -9 meter 1  m = 10 -6 meter X-ray   ~ Ångstrom=0.1nm, ~ 10 18 –10 19 Hz  1 Å = 10 -10 meter (size of an atom)

Light in a Medium When light enters a medium, it wiggles the charges in it, which in turn radiates more light. All add up to a single light wave traveling in the media (transparent). The oscillation frequency does not change. The speed of light is slower in a medium v=c/n (n: index of refraction, 1.33 inWater ) thus the wavelength becomes shorter!

Making E&M Radiation Find devices that wiggle charges! High frequencies: atoms and nuclei Intermediate frequencies: heating (motion of atoms), Temperature ~ frequency Your face: 10,000 nm Heating and cooking: 1000 nm Sun: 500 nm Low frequencies: electric circuits