What is light?

Summary of Ulysse Dormoy’s article “What Is Light?”

“It occurred to me that since I started posting articles and thoughts about light, I haven’t yet raised the question - ‘What is Light?’ – actually, in 30+ years in the lighting industry it's not a question I've challenged myself with.” – Ulysse Dormoy

Light exists in a state of duality. It behaves both as a wave and as a particle, a phenomenon known as wave–particle duality. Like the classic “chicken or egg” question, it challenges ideas of origin and cause. Scientifically, light has always been one phenomenon with two inseparable aspects. As a wave, it explains how light propagates and interacts with matter; as a particle, it exists as photons, discrete packets of energy that enable quantum-level interactions.

For centuries, scientists debated whether light was a wave or a particle, until early 20th-century quantum theory revealed it to be both. This insight, driven by Einstein and the rise of quantum mechanics, fundamentally reshaped physics, and it’s a surprisingly recent one.

In a biological context, this single phenomenon serves two purposes: perception and physiological interaction. Life on Earth evolved under the full solar spectrum, long before vision existed. Light-sensitive cells appeared around 600 million years ago, with true vision evolving roughly 60 million years later. This suggests that light’s energetic, particle-based role may have preceded its visual one.

Photons, the particle aspect of light, are central to life. As Geoffrey Guy notes in his book Quantum Biology, the photon may be the most important entity in the story of quantum biology. Yet modern lighting practice largely ignores this, reducing light to a visual tool rather than recognising its full biological power.

Since the rise of LEDs, the focus has been almost exclusively on energy efficiency and lumens per watt. Light is treated as something that consumes energy, not something that provides it. Metrics for humans remain visual (lux and lumens), while plants receive energetic metrics (PPFD), solar energy is measured in kilowatt-hours, and food is measured in calories. Why don’t we measure light’s energetic contribution to human wellbeing?

Research into quantum biology highlights that different parts of the spectrum play different roles. Visible light fuels life through photosynthesis, while near- and far-infrared light supports mitochondrial function, helping cells produce ATP efficiently, like lubrication for the body’s engine. Regular exposure to these photons may help maintain balance, resilience, and long-term health.

Modern lighting and glazing, in the pursuit of efficiency, have effectively “ultra-processed” light, stripping out wavelengths that were deemed inefficient but are biologically vital. Much like ultra-processed food, our indoor light environments may feed us visually but fail to nourish us energetically, especially given that we now spend over 90% of our time indoors.

Light’s duality underpins not just vision, but health and vitality. The built environment has been designed almost exclusively for perception, neglecting light’s role in human sustainability. With ageing populations and longer working lives, maintaining health and productivity will be critical, and light has a role to play alongside nutrition, movement, oxygen, and sleep.

We’re unlikely to reverse the trend of indoor living. Instead, we need spaces that help restore vitality, through thoughtful design, strategic exposure to biologically meaningful light, or simply by stepping outside for a daily photon shower.

Light is not just for vision.

It is for health, vitality, and human sustainability.