Electromagnetic energy is the compression and rarefaction of waves of magnetic-fields and electric-currents traveling through the vacuum of space or atmospheres of planets.  The sun is a source of electromagnetic energy.  The sun generates magnetic-fields that discharge electric-currents, which produce magnetic-fields that discharge electric-currents.  This is the compression-rarefaction cycle of electromagnetic energy.

The electromagnetic spectrum includes gamma rays, x-rays, ultraviolet light, visible light, infrared light-heat, microwaves, and radio.  Visible light and radio occupy a small fraction of the electromagnetic spectrum.  

As sound and the ocean wave, wavelength, frequency, amplitude, and subjective intensity (if applicable) also describes electromagnetic energy. 

Handbook Note:
Nano = one billionth  or 1/1,000,000,000
Trillion = 1,000,000,000,000

Visible light is the compression and rarefaction of visible-electromagnetic waves.  An initiating disturbance such as the sun or an artificial light source starts the cycle of visible electromagnetic energy.  The electro-magnetic-cycle continues until it terminates in our eyes or fades away.

White light projected through a prism separates into a rainbow of all the colors of visible light.

- The wavelengths of visible light range

   from 400nm to 700nm. 

- The wavelength of the
   color red is 570nm, green 540nm, blue 440nm. 

- Invisible light with wavelengths
  shorter than 400nm is the ultraviolet spectrum.

- Wavelengths longer than
   770nm is the infrared spectrum.  

The frequency of light measures in the trillions of cycles per second.  A frequency range of 400 to 750 trillion cycles per second.  Note, this is a very narrow range when compared to the ten-octave range of the ear. 

The amplitude of light is its intensity, measured by its directional radiating power of luminance.  Luminance, measured in units of the lumen, ranges from 0 to more than 12000 lumens. 

Use the following as a reference: 
- The amplitude of Indoor artificial light ranges
   from 400 to 600 lumens.
- Outdoor brightness ranges
   from 1000 to 12000 lumens.
- Our eyes are comfortable to about 3,500 lumens.
- 4,000 lumens becomes glare.
- At 10,000 lumens our eyes block light.
- 12,000 lumens is the amplitude of snow blindness. 

The subjective intensity of the light to the human eye is brightness.  Brightness is to light as loudness is to sound.  As the ear to sound, the eye is not equally sensitive to the entire bandwidth of light.  It is very sensitive to the wavelength of green light, less sensitive to red light, and even less to blue light.  Furthermore, the amplitude of light creates two states of vision - scotopic and photopic.

The scotopic state is our adapted night vision.  It involves the rods of the eye's retina, which are very sensitive to the intensity of light. 
But their sensitivity is limited to the shorter blue colored wavelengths. 

The photopic daylight state involves the cones of the eye's retina,
which are very sensitive to the entire spectrum of color while much-less sensitive to the intensity of light.

Handbook Note:
Speed of light and radio = 186,000 miles/second
Speed of sound = 1130ft/second 

Radio is the compression and rarefaction of invisible electromagnetic waves of energy.  The disturbance of radio broadcast antenna "sparks" initiate this cycle.  The cycle terminates at a receiving antenna or fades away. 

A radio broadcast begins with the continuous controlled spark of a modulating electric current in a copper wire of a broadcast antenna.  The modulating current of electrical spark generates cycles of electromagnetic waves.  The sparking-waves cause electrons in a receiving antenna to budge, dislodge, and regenerate new radio cycles of electromagnetic current.  

Broadcast radio covers a frequency range of thousands of cycles per second as AM radio to billions of cycles per second as satellite TV. 
The typical car radio displays the broadcast frequency on its front panel -- AM in kHz and FM in MHz.

The wavelength of radio ranges from a few hundred yards as AM radio to a fraction of an inch as satellite TV.

The power or amplitude of radio's electromagnetic energy, measured in watts, is equal to the flow of electrical current in amps multiplied by its pressure in volts.

A subjective response to electromagnetic radio does not apply as it did with sound and light because humans cannot see or hear electromagnetic radio waves.

Don't change that channel.  We will return to the exciting conclusion of radio in chapter 3 following these messages.  Please stay tuned.

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