From an atom to the colour you see

Colour is a long way from an atom. Electrons respond to light. That response sets how much a material bends light. Layers of material reflect some wavelengths and pass others. An eye turns what arrives into a colour.

Usually each of those steps belongs to a different field, with its own software. The engine does them as one chain:

1. The electrons of the atoms or molecules settle into their state.
2. From that state, how strongly they respond to light of each wavelength.
3. From that response, the refractive index of the bulk material.
4. From the refractive index, what a stack of thin layers reflects.
5. From the reflected light, the colour a person sees.

It runs both ways

Every step can be run forwards and backwards. Forwards: here are the atoms, what colour is it? Backwards: here is the colour I want, what would have to be true of the material?

Running it backwards is what turns a prediction engine into a design engine.

What has been checked

As of the last write-up (May 2026):

• The chain reproduces a well-known ultraviolet absorption line of helium to within about 2 percent in energy.
• The layer step gives the textbook-exact answers for an anti-reflection coating and for a mirror made of stacked layers.
• Each step, run forwards and then backwards, returns to where it started.

These are checks against cases where the answer is already known. They show the chain is sound. They are not yet new discoveries.

What would tell us more

The same chain that sets the colour of a butterfly wing also shapes the light leaving a star’s atmosphere. A demanding test is to take a measurement made in a laboratory on Earth and predict what an astronomer should see.