Visible Light Spectrum

That's hard to discuss, because the electromagnetic spectrum has no ends.

If you name a frequency, then no matter how low it is, I can name a lower one,

and no matter how high it is, I can name a higher one. So, it's easy to describe

the size of the visible portion, but it's hard to describe the size of the full EM spectrum.

So let's just talk about the size of the part of the E&M spectrum that humans

use for radio communication, and ignore all the rest of it ... the infra-red, x-rays,

ultraviolet, gamma rays, all that other stuff. Only the part that we know how to

generate and modulate with very precise frequency control, and transmit and

receive over significant distances.

That's the portion of the E&M spectrum with frequencies from about 60 KHz

to about 100 GHz (wavelengths from about 3 millimeters to 5 kilometers).

'Linearly', that's a range of about 100 GHz bottom-to-top. But a much better

way to talk about parts of the E&M spectrum is logarithmically ... how many

'octaves' (doublings) or 'decades' (multiplied by 10) they cover.

Measured that way, our use of radio spans about 21 octaves, or about 6.2 decades.

Now we're ready to go look up the frequency/wavelength range of visible light.

A typical human eye will respond to wavelengths from about 390 to 750 nm

(3.9 to 7.5 x 10-4 millimeters). In terms of frequency, this corresponds to a band

in the vicinity of 400-770 THz (400,000 to 770,000 GHz).

That makes the visible range about 0.95 octave, or about 0.28 decade ... only

about 41/2% as wide as the range of wavelengths we use for radio communication!

Now, for fun, we'll try and include those other E&M phenomena that we've been ignoring.

We'll still have to decide where the ends of the spectrum are, because it really doesn't

have any.

For the bottom frequency, let's take 60 Hz. That's the small amount of RF that

radiates from power lines, which we always ignore. The wavelength is about

5,000 kilometers. (!)

Let's take gamma radiation for the top end ... the stuff generated in nuclear

decay, supernovas, black holes, that sort of thing. Dangerous stuff because

of its high energy. We're still here only because Earth's atmosphere absorbs

most of the gamma radiation from space, and not much of it ever reaches the

ground. Astronauts have to be shielded from it.

Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and

wavelength less than 10 picometers (less than the diameter of an atom.)

So now, our 'expanded' range of electromagnetic spectrum covers 57.2 octaves,

or 17.2 decades, and the range of visible light is about 1.6% as wide as that.

Bottom line . . . we don't actually "see" a whole lot of the E&M spectrum, but

we know how to build instruments that detect the parts we can't see.

The 7 colors of the visible spectrum are Red, Orange, Yellow, Green, Blue, Indigo, and Violet.

A good way to remember this is by the name Roy G Biv, because each letter of that name is starting letter of the spectrum colors. (For Example, the y in Roy stands for yellow.)