Colors Are In the Brain of the Beholder: Physics of Wavelengths

We see the world as vibrant with colors: the sky appears blue, the grass seems green, and flowers exhibit the full spectrum of existing hues and shades. Yet, when night falls, these colors vanish, replaced by a dull grey. What becomes of them? The truth is, they never were there in the first place. Colors don't exist in nature. They reside in our minds, which convert light wavelengths into visual perceptions that help us interact with the external world.

Fig. 1. Colors do not exist in nature; they are visual representations of wavelengths, constructed by the brain. At night, when the brain receives too little information, the vivid coloration of butterflies fades into greyscale.

What Newton’s prism revealed

The first indication that colors are not quite what they seem came when Isaac Newton passed a beam of sunlight through a prism, splitting it into the colors of the rainbow (Fig. 2). If light is white, then where do these colors come from?

To test whether the prism itself was somehow producing these colors, Newton performed a second experiment. He used one prism to split white light into a spectrum, then allowed only one narrow color, such as red, to pass through a second prism. The red light remained red: the prism neither split it into new colors nor transformed it back into white. This showed that the colors were intrinsic to the white light, rather than the prism somehow altering the light's whiteness.

Fig. 2. White light passing through a prism separates into the visible spectrum, traditionally divided by Newton into seven colors. The separation occurs because different wavelengths are refracted by different amounts.

The light we see as white was found to comprise all the colors of the visible spectrum. Prisms don't modify the properties of light; instead, they reveal them through refraction. We think of white as the absence of color, a blank canvas to which colors can be added. So, recognizing that white is the opposite of that, it is the holder of all colors, can be challenging. But what can be separated can be reassembled. If we combine the colors of the spectrum, or even the boundary red and blue with the middle green, we get back to white (Fig. 3).

Fig. 3. Additive color mixing. Primary colors: red, green, and blue produce white. Where two primaries overlap, they produce secondary colors, where yellow = red + green, cyan = green + blue, and magenta = red + blue.

Newton demonstrated that white light is composite. He also explained the sequence of colors in the spectrum based on their "refrangibility," a term he used to refer to the tendency of colors to bend as they pass through a medium. Red and violet are at the ends of the spectrum because red bends the least and violet the most (Fig. 2). It was only later, when Thomas Young discovered that light can behave as a wave, that refrangibility could be understood in terms of wavelength.

From colors to wavelengths

During Newton's era, light was considered purely corpuscular. The wave nature of light was revealed when Thomas Young performed his famous double-slit experiment. In 1801, he passed light through two narrow slits, producing an interference pattern of alternating bright and dark fringes (Fig. 4). This pattern is difficult to explain if light is merely a stream of particles. Still, it follows naturally if light is a wave. Where two wave crests meet, they reinforce each other; where a crest meets a trough, they cancel.

Fig. 4. Young’s double-slit experiment showed that light behaves as a wave. When coherent light passes through two narrow slits, the emerging waves interfere, producing alternating bright and dark fringes on the screen.

The spacing of these fringes depends on the wavelength of the light. By studying interference patterns, Young and later scientists could measure the wavelengths associated with different regions of the visible spectrum (Fig. 5). Red light, which bends the least, was found to have the longest visible wavelengths, around λ ≈ 700 nm, while violet light, which bends the most, has the shortest visible wavelengths, around λ ≈ 400 nm. Green light lies midway between them at about λ ≈ 550 nm.

Fig. 5. The visible spectrum spanning red to violet. As wavelength (λ) decreases, frequency (f) increases.

This was a crucial step toward describing the spectrum in terms of measurable physical quantities. Newton had divided the spectrum into seven colors: red, orange, yellow, green, blue, indigo, and violet. But when we evaluate the spectrum by wavelength, this division begins to dissolve. There are no sharp borders where red ends and orange begins, or where blue turns into violet. Instead, there is a continuous, seamless transition of wavelengths, frequencies, and the associated energy.

Young–Helmholtz trichromatic theory

Once colors are linked to wavelengths, it becomes tempting to say that they can be found in nature. However, a deeper clue emerged. Two lights with vastly different spectral compositions can appear identical; these are known as metamers. For instance, the monochromatic yellow light can also be produced by a dichromatic combination of red and green light (Fig. 6). If our visual system cannot distinguish between two physically different wavelength combinations, then how can color be an objective feature of the natural world?

Fig. 6. Dichromatic yellow is produced by overlapping red and green light. To the human eye, this can appear the same as monochromatic yellow light because both can stimulate the cone cells in equivalent ways, a perceptual effect called metamerism.

This led to the Young–Helmholtz trichromatic theory. The photoreceptors, which capture light, are positioned in the retina at the back of the eyeball (Fig. 7). The ones involved in color recognition are called cones. Rather than having a separate receptor for each wavelength, the cones are divided into three categories, each responsible for a different part of the spectrum. L-cones are sensitive to long wavelengths (reds), M-cones respond to medium wavelengths (greens), and S-cones detect short wavelengths (blues).

Fig. 7. The Basic eye anatomy showing how light enters the eye and reaches the retina.

As light enters the eye, cones respond to their respective wavelength ranges, converting the excitation into electrical signals, which are then transmitted to the brain through the optic nerve. The brain evaluates these signals and interprets them as colors. This mechanism explains why the three primaries: red, green, and blue, can reproduce such a wide range of perceived colors.

The real properties are wavelengths

The strongest evidence that the brain creates color came from neurology. came from neurology. Some people with damage to particular regions of the visual cortex (Fig. 8) can lose color perception while still retaining other aspects of vision, such as the ability to see shapes and movement. This indicates that the photoreceptors in the retina continue to gather physical data about light and relay it to the brain. Yet, without the cortex’s ability to process the information about wavelengths, the colored picture of the world does not emerge.

Fig. 8. The visual cortex at the back of the brain, with the visual pathway from the eye through the optic nerve and thalamus.

This brain condition, known as cerebral achromatopsia, shows that colors are not direct copies of the external world. They are useful interpretations generated by the visual system, helping us distinguish objects, detect changes, and navigate our surroundings. However, nature does not speak in reds, greens, and blues; it speaks in wavelengths, frequencies, and energies. Our brains convert these physical signals into color categories. Because human biological needs influence this conversion, it does not always match physical reality.

Certain colors demonstrate this mismatch particularly well. Purple and magenta, for example, do not correspond to any single wavelength in the visible spectrum. There is no purple or magenta ray in a rainbow between red and violet. These colors appear when long and short wavelengths actively stimulate the retina, while the middle wavelengths remain largely absent. The brain compensates for the gap by creating imaginary signals, generating color combinations from wavelengths that don't even exist in nature.

The brain's ability to create colors that have no place on the wavelength line of the visible spectrum can be further illustrated by brown and grey. These colors are contextual perceptions produced by brightness, contrast, and surrounding colors. In this sense, some of the colors we experience as perfectly real have no physical backup in objective reality. They are constructs of the visual system, adjusting the language of nature to support human drive for survival.

Why do colors disappear at night?

This brings us back to the question we began with: why does the vibrant world of colors begin to crumble after sunset? The cones need bright light; they don't function well in dim conditions. At night, another class of photoreceptors, rods, takes over. Rods assist the brain in forming an image of the world based on light intensity rather than wavelength. By detecting differences in brightness across the visual field, rods provide the information needed to recognize the objects' shapes and movements.

Rods are incredibly sensitive to brightness, which makes them essential for night vision. However, they cannot distinguish between wavelengths and can't paint the world in color. This doesn't mean that wavelengths vanish. Even in low light, objects continue to absorb and reflect it. The problem is that the brain receives too little wavelength-specific information, and the colorful world collapses into shades of grey.

Wavelengths are fundamental features of physical reality, existing independently of our presence or awareness. Colors, by contrast, are the products of our visual system, existing only when our brain has sufficient information to keep its channels active. The moment we take our eyes off the sky and grass, their blueness and greenness disappear. What remains are wavelengths, frequencies, and energies, the physical quantities of objective reality that exist independently of an observer.