### Chapter 2 - Goethe's Theory of Colour

By Johny Jagannath

Goethe was a famous German poet who made some very important observations on the behavior of light. He was specifically interested in knowing how a prism splits white light into its constituent colors. So he performed several experiments with prism and white light. During his experiments, he noticed that colors always originated at the boundary where darkness and light met.

To understand what Goethe means please view this video for a few seconds. Is the runway made of colored stripes or is this an illusion? As the first model begins her catwalk, you will notice that the runway has no colors on it, at all. We only see a pattern of Black and White, stripes on it. This becomes clearer when the camera zooms in on some models' shoes (or handbags) to reveal a perfectly 'black' and 'white' runway.

But we do see colors, on the runway from a certain distance. So why do we see these colors on the runway? Goethe's answer: Colors arise only in the presence of 'darkness'. In the context of the runway, the 'darkness' that Goethe is referring to is the numerous black stripes on the runway. Without those black stripes, we'd see no colors on the runway.

Remember that Goethe's observations are scientific and are repeatable in a lab. It's not like he was a poet who did not believe in the scientific method. Therefore, Goethe's observations must hold good in any scenario where colors arise, in the universe. And they do.
Since the color phenomenon relies on the adjacency of light and dark, there are two ways to produce a spectrum: with a light beam in a dark room, and with a dark beam (i.e. a shadow) in a light room. [The animation below shows both, light and dark spectrum. Wikipedia]
 Light and dark spectra – when coloured edges overlap in a light spectrum, green results; when they overlap in a dark spectrum, purple colours result.
Therefore, I will extend Goethe's logic of "adjacency of light and dark for colors to arise" to a prism at a 'molecular' level to explain why a prism splits white light into colors.

 Glass molecules
To the left you can see the picture of the molecular structure of glass. As you can see it looks like a mesh. And there are millions of these meshes in a piece of glass. So, what happens when you view light through a few meshes?

The picture below answers the question. The picture is of a mesh-chair. It has two meshes. One on the front and the other on the back. As you can see, there are clear interference patterns on it.

 Interference on mesh chair
Similarly, a prism owing to its molecular geometry (bunch of meshes), creates a runway like situation where the black stripes are created by the molecules of glass that block out light, and the bright stripes, when light slips through the meshes. [Just as the mesh-chair is doing in the picture to the right.]

And per Goethe, where there are dark and light patterns, colors arise. This is why a prism appears to split white light into colors. In reality white light has no colors in it, at all.

Also, the mesh analogy applies to all objects that are made of molecules. And therefore all objects are capable of producing a unique pattern of interference based on their own molecular structure, which in turn determines the kind of colors that originate from that object. That's how we see the world in color.

If it weren't for the mesh-like-molecular geometry we would all be watching a world or the universe in black and white. And that would have been a pretty sad world, not only from an aesthetic point of view but also from the stand point of science because everything we know about distant objects is via its light and color.

Therefore, any interference experiment with white light produces colors, such as the soap bubble picture below. Click here to see more, images of white light interference.

 Thin film interference (It's a soap bubble, not Jupiter.)
Now we are left with the task of determining the ratio of darkness to light, that will produce a corresponding color, in a photon. For e.g., a photon with 10% darkness and 90% light would look Green or Yellow. Another photon with 60% light and 40% darkness would look Red or Orange. Once this is determined for all colors, it needs to be related to polarization, and spectral lines. By now, the reader probably understands how this is going to happen. This will be the focus of my next post.