The Physics of Colour: Why the Sky Is Blue and Sunsets Are Red

What You See Is Not What Is There

A red apple does not emit red light. It absorbs most wavelengths of white sunlight and reflects predominantly those around 650 nanometres — which your eye and brain interpret as red. Change the illumination (use blue-only light) and the apple appears nearly black. The “colour” of the apple is not intrinsic to it. It is a product of the interaction between light, matter, and the human visual system.

Understanding colour requires both physics — the behaviour of electromagnetic radiation — and biology. The physics determines which wavelengths exist and how they interact with matter. The biology determines which wavelengths we detect and how the brain constructs the experience of colour.

The Electromagnetic Spectrum

Visible light is electromagnetic radiation with wavelengths between approximately 380 nm (violet) and 700 nm (red). This is a tiny sliver — less than one octave — of the electromagnetic spectrum, which spans from radio waves (wavelengths of kilometres) to gamma rays (wavelengths smaller than atomic nuclei).

Each wavelength within the visible range corresponds to a spectral colour: violet (~380–450 nm), blue (~450–495 nm), green (~495–570 nm), yellow (~570–590 nm), orange (~590–620 nm), red (~620–700 nm). Beyond red lies infrared; beyond violet lies ultraviolet. Both are invisible to the human eye, though many animals can see into the ultraviolet.

White light — sunlight, for example — is a mixture of all visible wavelengths. When Isaac Newton passed white light through a glass prism in 1666, it separated into a continuous rainbow of spectral colours because the glass refracts (bends) each wavelength by a slightly different angle. This dispersion proved that white light is composite, not fundamental.

How Objects Get Their Colour

The colour of an object depends on which wavelengths it absorbs and which it reflects or transmits. This is determined by the electronic structure of the material — specifically, which quantum mechanical energy transitions its electrons can undergo.

Absorption and reflection — When a photon strikes a material, it can be absorbed if its energy matches an available electronic transition. Chlorophyll absorbs red and blue light strongly (using the energy for photosynthesis) but reflects green — which is why plants appear green.

Pigments and dyes — The specific wavelengths a pigment absorbs depend on its molecular structure. Conjugated organic molecules (alternating single and double carbon bonds) absorb visible light because their delocalised electrons have energy gaps in the visible range. The more extended the conjugation, the longer the wavelengths absorbed — which is why carotene (11 conjugated double bonds) is orange while lycopene (13 double bonds) is red.

Structural colour — Some colours arise not from pigments but from the physical structure of a surface. The iridescent blue of a morpho butterfly’s wing comes from nanoscale ridges that create thin-film interference — constructively reinforcing blue wavelengths while cancelling others. No blue pigment is present. Opals, peacock feathers, and CDs also display structural colour through interference and diffraction.

Incandescence — Hot objects emit thermal radiation across a spectrum determined by their temperature. As temperature rises, the peak emission shifts from infrared through red, orange, yellow, and white toward blue — which is why a candle flame is yellowish, a light bulb filament is warm white, and a star like Sirius appears blue-white. This is described by Planck’s blackbody radiation law.

Rayleigh Scattering: The Blue Sky

The blue sky is a direct consequence of how electromagnetic radiation interacts with particles much smaller than its wavelength.

Lord Rayleigh showed in 1871 that the intensity of light scattered by molecules is proportional to 1/λ⁴ — the fourth power of the inverse wavelength. Blue light (λ ≈ 470 nm) is scattered approximately 5.5 times more intensely than red light (λ ≈ 650 nm).

When sunlight enters the atmosphere, blue wavelengths are preferentially scattered in all directions by nitrogen and oxygen molecules. When you look at the sky away from the Sun, you see this scattered blue light arriving from all parts of the atmosphere. The Sun itself appears slightly yellowish because some of its blue light has been scattered away.

At sunrise and sunset, sunlight travels through a much longer atmospheric path — up to 40 times longer than at noon. Over this extended path, virtually all blue and green light is scattered out, leaving only the red and orange wavelengths to reach the observer directly. Particles like dust, smoke, and volcanic aerosols enhance the scattering of shorter wavelengths and produce particularly vivid red and purple sunsets.

Refraction and Rainbows

A rainbow is a natural spectrometer. Sunlight enters a raindrop, refracts at the air-water interface, reflects off the back of the drop, and refracts again as it exits. Because the refractive index of water varies slightly with wavelength (dispersion), each colour exits at a slightly different angle — red at about 42° and violet at about 40° from the anti-solar point.

Each raindrop disperses the full spectrum, but an observer sees only one colour from each drop — the colour corresponding to the angle at which that drop sits relative to the observer’s line of sight. The collective effect of millions of drops at different angles produces the arc of colours we see as a rainbow.

A secondary rainbow (sometimes visible outside the primary) results from two internal reflections. The extra reflection reverses the colour order (red on the inside, violet on the outside) and makes the secondary bow fainter. The dark band between the primary and secondary rainbows — Alexander’s dark band — is the region where no light from single or double reflections reaches the observer.

The Human Eye: Three Channels of Colour

The human retina contains roughly 6 million cone cells of three types, each sensitive to a different range of wavelengths: S-cones (short wavelength, peaking around 420 nm — blue), M-cones (medium, peaking around 530 nm — green), and L-cones (long, peaking around 560 nm — red). The brain interprets colour based on the relative stimulation of these three cone types.

This trichromatic system means that our colour perception is a lossy compression of the electromagnetic spectrum. A pure spectral yellow at 580 nm stimulates both L- and M-cones in a particular ratio. A mixture of red (650 nm) and green (530 nm) light can produce the same ratio — and therefore the same perceived colour — despite being physically completely different. This equivalence is called metamerism.

Some colours we perceive have no single-wavelength equivalent. Magenta — the “colour” between red and violet on the colour wheel — does not appear in the rainbow. It is the brain’s response to simultaneous stimulation of L-cones (red) and S-cones (blue) with little M-cone (green) stimulation. Magenta is a neurological construct, not a spectral reality.

Colour in the Quantum World

The connection between colour and quantum mechanics runs deep. The specific wavelengths emitted and absorbed by atoms are determined by the quantised energy levels of their electrons. Each element has a unique emission spectrum — a barcode of spectral lines — which is why sodium street lamps are yellow (the sodium D-lines at 589 nm) and neon signs are red (neon’s dominant emission lines around 640 nm).

This is the same physics that powers spectroscopy — the technique that reveals the composition of distant stars, exoplanet atmospheres, and interstellar gas clouds. Every colour of light an object emits or absorbs carries information about its atomic and molecular structure.

The physics of colour connects the quantum behaviour of electrons in atoms to the everyday experience of a sunset, a rainbow, or the blue sky overhead. What seems subjective and aesthetic is, at its foundation, electromagnetic radiation obeying quantum rules — physics that you can see.