What is Light? The Physics Every Designer Should Know

Light is one of those things everyone uses and almost nobody understands. That gap — between familiarity and knowledge — is where poor lighting decisions are made every day.

So let us start at the beginning.

Light is electromagnetic radiation. It is energy travelling as waves at 300,000 kilometres per second. What we call visible light is a remarkably narrow band of the electromagnetic spectrum, occupying wavelengths between approximately 380 and 700 nanometres. Everything you see — every colour, every shadow, every mood a room creates — exists within that sliver.

Here is the part that changes how you think about your work.

Light has no colour of its own. Colour is your brain's interpretation of different wavelengths hitting your eye. Violet sits at the short end, around 400nm. Red sits at the long end, around 700nm. When you choose a light source, you are not just choosing brightness. You are choosing which wavelengths are present — and that determines how every material, every finish, and every face in your space will appear.

This is why two rooms with identical lux levels can feel completely different. One has a light source with a rich, full spectrum. The other has gaps — wavelengths missing or suppressed — and the space feels flat, washed out, or simply wrong in a way nobody can quite name.

For architects, this means the light source is a material decision, not a services decision. For interior designers, it means finish selection and light source selection must happen together. For contractors, it means the specification matters — substituting a fitting without understanding its spectral output is not a neutral act. For homeowners, it means that uncomfortable feeling in a room is often not the furniture or the paint. It is the light.

Understanding what light is does not make lighting decisions more complicated. It makes them more deliberate. And deliberate decisions produce better spaces.

Before you design with light, understand what light is. Everything else follows from that.