Visible light refers to a segment of electromagnetic waves. Electromagnetic waves are divided into different bands according to their wavelengths. Generally speaking, the band with wavelengths ranging from 380 nm to 700 nm is defined as visible light, since electromagnetic waves within this range can be perceived by human eyes in the form of light. The exact wavelength range perceived may vary slightly among individuals, yet it basically falls within the above interval.

So why is visible light "visible"? In other words, why can human eyes detect electromagnetic waves of this specific wavelength band?

The most straightforward answer lies in the molecular mechanism underlying human vision. The human eye is structurally analogous to a high-precision camera. Light travels through the transparent cornea and the pupil surrounded by the iris, refracts via the lens, and forms a spatially distributed image on the retina. Two types of photoreceptor cells reside on the retina: rod cells, which primarily detect light intensity, and cone cells, which are responsible for color perception. These cells constitute the cellular foundation of vision formation.

Though rod cells and cone cells exhibit slight differences in light responsiveness, they share an essentially identical photoresponse mechanism. Take rhodopsin on rod cells as an example: it consists of an integral seven-transmembrane protein (opsin) embedded in the cell membrane and a retinal prosthetic group. As a type of G protein-coupled receptor (GPCR), opsin covalently binds the retinal cofactor to a lysine residue on its seventh transmembrane α-helix segment.

Retinal is derived from the oxidation of vitamin A, with one molecule of vitamin A yielding one retinal molecule upon oxidation. Retinal exists in two configurations: all-trans and 11-cis. The 11-cis form is the native configuration bound to opsin. When exposed to visible light (approximately 500 nm wavelength for rhodopsin), 11-cis retinal is isomerized into the all-trans configuration, causing retinal to detach from opsin. This dissociation triggers conformational changes in rhodopsin. Subsequent signal transduction alters ion potentials across the cell membrane, generating electrical nerve signals. These signals are transmitted to the brain via the optic nerve, ultimately producing visual perception.

Therefore, from the perspective of visual molecular mechanisms, electromagnetic waves in the visible spectrum are detectable to humans precisely because the configurational isomerization reaction of retinal is triggered by this exact wavelength band.

This raises a follow-up question: Why does vision rely on retinal molecules in the first place? Could vision evolve to respond to other electromagnetic bands if different photoreactive molecules were adopted?

To address this question, we need to examine the evolutionary development of biological vision. Plants generally lack vision but possess photosensitivity, most prominently demonstrated through photosynthesis, alongside phototropic behaviors observed in certain species such as sunflowers.

In contrast, the vast majority of animals possess vision of varying sophistication. The most primitive animal visual organ is the eyespot of paramecium, located at the base of its flagellum. It only senses variations in visible light intensity. As paramecium is a single-celled organism, its eyespot is merely a concentrated cluster of light-sensitive proteins, rather than a fully differentiated visual organ.

Following the emergence of multicellular organisms, eyes diversified dramatically across nature. A vast array of eye structures evolved, including unicellular eyes in earthworms, cup eyes in jellyfish, vesicular eyes in snails, compound eyes in insects, and lens eyes shared by vertebrates and a small number of invertebrates such as octopuses. Biologists once hypothesized that animal eyes evolved independently on multiple occasions. However, recent molecular evolutionary research has confirmed that eye development across all animal lineages is regulated by the highly conserved, homologous Pax 6 gene family, strongly supporting a single evolutionary origin for animal eyes.

If eyes evolved once, their core light-sensing molecules should also share a single evolutionary origin. In fact, aside from a few evolutionarily basal organisms that utilize flavins as photoreceptors, nearly all animals adopt seven-transmembrane proteins paired with retinal-based molecules as light-detecting components. Derived from partial oxidation of vitamin A, retinal can be further synthesized from β-carotene, a naturally widespread pigment in plants. Its clear synthetic pathway and metabolic origin make it a logical candidate selected by biological evolution as the primary photoreceptive molecule.

Nevertheless, the above analysis only explains why retinal is evolutionarily favorable, not an absolute necessity. Are there no alternative molecules capable of fulfilling photoreceptive functions?

Let us approach this question hypothetically: if we were to design a biological photoreceptive system, what molecule would we choose, and which electromagnetic band would it respond to?

First, gamma rays and high-energy X-rays with the shortest wavelengths must be ruled out. Their extremely high energy rapidly induces molecular ionization, decomposition, and even nuclear excitation.

Deep ultraviolet radiation and soft X-rays primarily excite inner-shell or high-energy valence electrons, generating highly unstable molecular excited states that cannot reliably transmit biological information at ambient temperatures in aqueous solutions or air.

Infrared and microwave radiation couple mainly with molecular vibrational, rotational and translational motion. Such movements manifest predominantly as random thermal motion, making precise signal encoding and transmission unfeasible.

Electromagnetic waves with longer wavelengths, such as medium and long radio waves, operate at length scales exceeding the detection range of individual molecules, rendering them unsuitable for cell-based biological photoreception. Humans have, however, harnessed these bands (including microwaves) to develop communication devices such as radios, mobile phones and televisions, which rely on integrated circuits rather than light-sensitive biomolecules.

This evaluation leads to a clear conclusion: if cells are to employ a molecular-level photoreactive mechanism to respond to electromagnetic radiation, the visible spectrum is the optimal candidate. This band corresponds to outer-shell electron excitation energies within molecular electronic spectra, slightly lower than the energy of common chemical bonds that sustain life (C-C, C-H, C=O, C-N bonds, etc.). It is energetic enough to trigger photoresponsive reactions in dynamic double bonds (such as the photosensitive cis-trans isomerization at the 11-position double bond in retinal) for effective signal transmission, yet insufficient to damage stable essential biological chemical bonds.

We may also analyze the ambient electromagnetic radiation environment on Earth. Solar radiation overwhelmingly dominates planetary electromagnetic input; radioactive decay within the Earth and cosmic rays are negligible by comparison. Solar radiation approximates blackbody radiation, with its spectral distribution governed by the surface temperature of the Sun. Spectral measurements confirm the Sun has a surface temperature of roughly 5250 °C, with the peak irradiance falling within the visible spectrum at the outer edge of Earth’s atmosphere.

Atmospheric gases including nitrogen, oxygen, water vapor and carbon dioxide exhibit strong absorption in the infrared range yet minimal absorption across visible wavelengths. As a result, the peak intensity of solar radiation reaching sea level still lies within the visible band. Life originated in oceanic environments, and seawater transmittance exerted a profound influence on the evolutionary selection of photoreceptive wavelengths. Coincidentally, liquid water features an absorption minimum near 800 nm, with strong absorption for deep ultraviolet and infrared wavelengths, while being nearly fully transparent to visible light.

Combining both the intrinsic properties of biomolecules and planetary environmental conditions, cell-based life systems must evolve light-sensing molecules tuned to the visible spectrum to respond to electromagnetic radiation. Even if retinal were not the chosen photoreceptor, evolution would favor alternative molecules or molecular assemblies with equivalent photochemical properties.

At this point, we arrive at a fundamental philosophical question: Why did cell-based life emerge on an Earth-like planet in the first place?