Does a rose look red when no one is looking? The physics of color perception

Does a rose look red when no one is looking? We see the world as vibrant with color: the sky appears blue, the grass seems green, and flowers display a wide variety of hues and shades. Yet when night falls, these colors disappear, leaving only dull shades of grey. Where do they go? The truth is, they were never there. Colors don't belong to objects. Color perception arises when our brains convert patterns of light wavelengths into visual experiences that help us interact with our environment.

Fig. 1.  Colors are not inherent in nature. Our brain creates them based on variations in wavelengths. At night, they shift to grayscale because the limited light greatly reduces the information sent to the brain.

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 a sunlight beam appears white, then where do these colors come from?

To determine whether the prism itself generated the colors, Newton carried out another experiment. He used one prism to separate white light into a spectrum and then directed a single red band through a second prism. The red light stayed red; the prism neither split it into new colors nor merged it back into white. This proved that the colors were intrinsic to the white light.

Fig. 2. When white light passes through a prism, it splits into the visible spectrum, which Newton characterized as a combination of seven colors.

Newton also explained the color sequence, using the term refrangibility, which refers to the tendency of colors to bend as they pass through a medium. Red and violet sit at the ends of the spectrum because red bends the least and violet the most. Only later, when Thomas Young discovered that light can behave as a wave, could refrangibility be understood in terms of wavelength.

From colors to wavelengths

In Newton's time, light was considered purely corpuscular. Thomas Young revealed the wave nature of light with 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. 3). This pattern is difficult to explain if light is merely a stream of particles. Yet it follows naturally if light is a wave.

Fig. 3. Young’s double-slit experiment. When coherent light passes through two narrow slits, a familiar interference pattern emerges on the screen, with alternating bright and dark fringes.

The spacing of these fringes depends on the light's wavelength. By studying interference patterns, Young and later scientists could measure the wavelengths associated with different regions of the visible spectrum (Fig. 4). Red light, which bends the least, was found to have the longest visible wavelengths of about 700 nm, while violet light, which bends the most, has the shortest wavelengths of about 400 nm. Green light falls between the two, with a wavelength of about 550 nm.

Fig. 4. The visible spectrum ranges from red to violet. Red has the longest wavelength (λ), and violet the shortest.

This was a crucial step toward describing the spectrum in measurable physical terms, such as wavelength. 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. No sharp borders mark where red ends and orange begins, or where blue turns into violet. Instead, wavelengths, frequencies, and associated energy transition continuously and seamlessly.

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, a mixture of red and green light can produce the same color percept as monochromatic yellow light (Fig. 5). If our vision can't distinguish between two colors with different physical characteristics, how can color be an objective feature of physical reality?

Fig. 5. We can't differentiate between dichromatic yellow, produced by combining red and green light, and monochromatic yellow because they both stimulate the cone cells in the same way.

This led to the Young–Helmholtz trichromatic theory. The photoreceptors responsible for color vision are called cones (Fig. 6). Rather than having a separate receptor for each wavelength, we have three cone types that together cover the entire spectrum. L-cones are most sensitive to longer wavelengths, M-cones to medium wavelengths, and S-cones to short wavelengths. This simple mechanism explains why the three primaries: red, green, and blue, can reproduce such a wide range of colors.

Fig. 6. The retina contains three types of cone photoreceptors: L, M, and S cones, which contribute to color vision. Cone density is greatest in the central retina, particularly within the macula.

Nature speaks in wavelengths

The strongest evidence that the brain plays a crucial role in creating color perception came from neurology. Some people with damage to certain regions of the visual cortex (Fig. 6) can lose the ability to perceive color while retaining other aspects of vision, including detailed central vision, which depends heavily on cones. This suggests that cones continue to respond to light and relay data to the brain. Yet without the cortex’s ability to interpret patterns of cone activity, the colored picture of the world does not emerge.

Fig. 6. The visual cortex at the back of the brain, with the visual pathway from the eye through the optic nerve and thalamus.

This demonstrates that nature communicates through wavelengths, frequencies, and energies rather than colors. Some colors don't even correspond to any wavelength in the visible spectrum. Purple and magenta emerge when light actively stimulates the L and S cones, while the M cones remain largely inactive. The brain compensates for this gap by imagining signals and generating colors that have no real basis in the physical world.

Why do colors disappear at night?

This brings us back to the question we began with: why do colors disappear after sunset? Cones require bright light and function poorly in dim conditions. At night, another class of photoreceptors, rods, becomes dominant. Rods help the brain form an image of the world based on light intensity rather than wavelength. By detecting differences in brightness across the visual field, they provide information that allows us to discern shapes and movement even when color vision fades.

However, a decline in cone activity doesn't mean that wavelengths disappear. Objects still reflect light, yet when too little reaches our eyes, the cone signals become too weak for the brain to process effectively. As a result, color perception fades and the vivid colors of the day give way to a predominantly grey landscape.

Colors aren't inherent in nature; our brain creates them to help distinguish objects and set them apart from their surroundings. As soon as we look away, the sky loses its blue shades, grass its green, and flowers their vibrant charm. What remains is just the blend of wavelengths they reflect. Without an observer, the world doesn't even display the grey tones we perceive in twilight. Color perception, then, is not a property of the world itself, but an experience our brains bring into being.