The world is filled with energy waves traveling through space, but human perception registers only a tiny fraction of these waves as light and color. Our ability to see allows us to navigate our environment, but it relies on a specific biological and atmospheric partnership that limits our vision to a narrow band of radiation. The majority of the universe’s energetic activity remains invisible to us, existing in ranges both shorter and longer than the light we perceive.
Context: The Electromagnetic Spectrum
The full range of energy waves is known as the electromagnetic spectrum (EMS), which spans from the longest radio waves to the most energetic gamma rays. All of these forms of radiation are fundamentally the same type of energy, differing only in their wavelength and frequency. Wavelength is the distance between successive peaks of a wave, and it is inversely related to frequency and energy.
To grasp the scale of the EMS, the portion that humans can see, visible light, is equivalent to a single key on a massive keyboard. The entire spectrum extends over a staggering range of more than 22 orders of magnitude in wavelength.
This vast spectrum includes X-rays, microwaves, and radio waves, none of which are visible to the human eye. Our planet’s atmosphere plays a role in what reaches us, filtering out many forms of high-energy radiation like gamma rays and most X-rays. The tiny section of the spectrum we perceive as light is one of the few ranges that can effectively penetrate the atmosphere and reach the Earth’s surface.
The Defined Range of Human Vision
The human visual system is tuned to a very specific slice of the EMS, typically cited as the range between approximately 380 and 750 nanometers (nm). This small range is what our eyes and brain interpret as the full spectrum of colors, from violet to red.
The shortest waves we can see, around 380 to 450 nm, correspond to the color violet, which possesses the highest energy within the visible spectrum. As the wavelength increases, the perceived color shifts through blue, green, and yellow. The longest visible wavelengths, extending from about 620 to 750 nm, are perceived as red light, which has the lowest energy in this range.
The reason we see this range is a combination of atmospheric transparency and the biology of the eye. The Earth’s atmosphere has an “optical window” that allows these specific wavelengths to pass through. Once inside the eye, these photons are absorbed by specialized light-sensitive cells, the rods and cones. These cells contain photopigments that chemically react only to these particular wavelengths, initiating the visual signal.
Beyond the Edges: Infrared and Ultraviolet
The visible spectrum is bordered by two invisible ranges: ultraviolet (UV) light on the short-wavelength side and infrared (IR) light on the long-wavelength side.
Ultraviolet (UV) Light
Humans cannot perceive UV light, despite its high energy, primarily because of a protective mechanism within the eye. The cornea and, more significantly, the lens of the eye absorb or filter out most UV radiation, preventing it from reaching the delicate retina at the back of the eye. This filtering action protects the photoreceptors from damage caused by the high-energy UV photons, which can be destructive to biological tissue.
Infrared (IR) Light
On the opposite end, we cannot perceive infrared light because its photons do not contain enough energy to trigger the necessary chemical reaction in the photopigments of our photoreceptors. The longer wavelengths of IR light, which begin just past 750 nm, simply fall short of this minimum energy threshold. Additionally, the vitreous humor, the water-based jelly filling the eyeball, absorbs many of the longer near-infrared wavelengths, further preventing them from reaching the retina.