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Calculating the exact percentage of the "entire" electromagnetic spectrum is actually a bit of a trick question because, theoretically, the spectrum is infinite. It ranges from radio waves as long as the universe to gamma rays smaller than an atom.
However, if we look at the range of wavelengths we typically interact with in physics (from roughly $10^4$ meters down to $10^{-15}$ meters), the math reveals just how "blind" we are.
The Numbers
Visible light exists in a very narrow band between approximately 380 nanometers and 750 nanometers.
The "Sliver": When you calculate the ratio of the visible bandwidth to the rest of the known spectrum, visible light accounts for about 0.0035%.
Energy-wise: If you look at it in terms of octaves (doubling of frequency), the visible spectrum is only one "octave" out of at least 60 to 70 octaves of known electromagnetic radiation.
Why is our "window" so small?
Evolution didn't pick this range by accident. There are two main reasons why our reality is limited to this specific sliver:
The Sun's Output: Our sun emits its peak power right in the visible light range. It’s the most "available" light on our planet.
The Atmospheric Window: Earth’s atmosphere is opaque to most radiation (like X-rays and most UV), but it is perfectly transparent to the visible light band. Our eyes evolved to use the only "clear" window to the outside world that we had.
Comparison to Other Ranges
To visualize how tiny that 0.0035% is, imagine the spectrum as a 1,000-mile-long road.
The Radio section would take up the first few hundred miles.
The X-ray and Gamma sections would cover the end.
Visible light would be less than the length of a single step on that 1,000-mile journey.
Would you like me to generate an image that visualizes this "1,000-mile road" analogy to show the scale difference?
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