The light given off by hydrogen in the tube or by the sun, or by a fire in a fire place, or by a red hot piece of metal is essentially the same thing as the radio waves that carry your favorite programs to you or the x-rays used to take pictures of your teeth. All are a part of the ELECTROMAGNETIC SPECTRUM.

The visible spectrum is just a small part of this range. Three quantities are needed to completely describe EMR:

• wavelength, λ (lambda) (m)
  
• frequency, ν (nu) (Hz or s^-1 )
  
• speed, c (3.0 x 10⁸ m/s)