Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio


Chapter Two
AV Physics
Page 3

Batting practice for the audio/video pro and a primer for the novice 



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Electromagnetic
Energy


          Electromagnetic energy is the compression and rarefaction of waves of magnetic fields and electric currents traveling through the vacuum of space or the atmospheres of planets. The sun is a source of electromagnetic energy.  It generates magnetic fields that discharge electric currents which produce magnetic fields that again discharge electric currents.  This is the cycle of electromagnetic energy.  
          T
he electromagnetic spectrum includes gamma rays, x-rays, ultraviolet light, visible light, infrared heat, microwaves, and radio.  Visible light and radio occupy a small fraction of the electromagnetic spectrum.
          The electromagnetic spectrum, as sound and ocean waves, can also be described
in terms of frequency, wavelength, amplitude, and if applicable subjective intensity.


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

              A nanometer or nm = a billionth of a meter
              A trillion = 1,000,000,000,000

Visible Light 
          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 cycle continues until it terminates in our eyes or fades away. 

          White light shone through a prism separates into a rainbow of all the colors of visible light.  The wavelengths of visible light range from 400 nm to 770 nm.  The wavelength of the color red is 570 nm, green 540 nm, and blue 440 nm.  Invisible light with wavelengths shorter than 400 nm is the ultraviolet spectrum.  Invisible light with wavelengths longer than 770 nm is the infrared spectrum.
          The frequency of visible light is measured in the trillion of cycles per second.  This is a range of 400 to 750 trillions of cycles per second.  Note that this is a very narrow range when compared to the ten-octave range of the ear.  
          The
amplitude of visible light is defined as its intensity.  The intensity of light is   measured by its directional radiating power or luminance.  Luminace is measured in units of lumen, which ranges from 0 to more than 12,000 lumen.  As a reference, consider the following:  
               - Indoor artificial light ranges in amplitude from 400 to 600 lumen.
               - Outdoor brightness ranges from 1000 to 12,000 lumen.
               - Your eyes are comfortable to a level of about 3,500 lumen.
               - 4,000 lumen becomes glare.
               - At 10,000 lumen our eyes block light.
               - 12,000 lumen is the amplitude of snow blindness.

          The percieved subjective intensity of light by the human eye is defined as brightness.  
Brightness is to light as loudness is to sound.  And similar to the loudness response of the ear -- the eye is not equally sensitive to the entire bandwidth of light.  The eye is most sensitive to the wavelength of green light, less sensitive to the wavelength of red, 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 sensitive to the entire spectrum of color.  But they are much less sensitive to the intensity of light.

                    Handbook Note:
                     Speed of Light and Radio = 186,000 miles/second .

                     Speed of Sound = 1130 ft/sec.

Invisible Radio

          Radio is the compression and rarefaction of invisible electromagnetic waves of energy. An initiating disturbance such as the sun or a radio broadcast antenna initiates this cycle that terminates at a receiving antenna or fades away.
          For example, a radio broadcast begins with modulating electric current in a copper wire, the broadcast antenna.  This generates cycles of electromagnetic waves.  The electromagnetic waves cause electrons in a receiving antenna to budge, dislodge, and regenerate cycles of electromagnetic current.  That is the invisible sequence of radio.
          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 is measured in watts.  A watt is equal to the flow of the electrical current in amps multiplied by its pressure in volts.  The issue of a subjective response to 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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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018