The Frequency of Visible Light

[Excerpted from Spectrums, by David Blatner]

Red light—that is, light that our human eyes perceive as red—has a frequency of about 420 THz (terahertz). That means electric fields and magnetic fields are flipping back and forth about 420 trillion times each second. If your car wheels revolved that quickly, you could drive from one end of our solar system to the other in the time it takes to blink an eye.

At that rate, each of those red-colored light waves is about 700 nm (700 billionths of a meter) long–about a tenth the size of a red blood cell–even smaller than a typical microscopic bacterium.

As you shorten the wavelength—that is, increase the frequency, so you get more waves per second—red becomes yellow, then green, then blue, and then, at about twice the frequency of red, purple. Speed up the frequency even more and the light changes, moving beyond what we can see into the ultraviolet (UV) range, then X-rays, then gamma rays.


  1. Richard t Shirley jr

    November 17, 2014

    If you broadcast these freqs would you not create a area of dead space that light couldnt shine or be seen I mean broadcast each individually to a single concentrated spot

  2. Katlyn

    February 5, 2015

    Hi this is excellent

  3. April 23, 2015

    THz is spelled “terahertz”, not “teraherz” ( ! ) and is named for Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves.

    One hertz simply means “one cycle per second”.

    Commonly used are Hz (Hertz), KHz (Kilohertz), MHz (Megahertz), GHz Gigahertz, and other larger expressions of cycles per second.

    — Doug Jones

    • admin

      April 23, 2015

      Great typo catch, Doug! Fixed. Yes, I discuss Hertz (and the unexpected reason it’s capitalized in abbreviations) in the book.

  4. Christopher Burgess

    May 21, 2015

    Interesting & maybe relevant that we see approx.one octave of electromagnet radiation. Maybe it could be used to calibrate the measurement of time.

  5. JT Fortner

    May 3, 2018

    What is the frequency and wavelength of purple/near UV light? Wikipedia’s answers vary from the different topics.

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