The Seven Colors of Light

Anyone who had studied physics probably knew this.

Visible light had different refractive indices in different environments, such as glass, air, and water.

These things had a general name: media.

As the theoretical study of optics became increasingly systematized, even many middle school students came to understand one principle:

Light could travel through a medium, but it did not need a medium to travel.

But they couldn't forget that Little Ox lived in 1665, when the study of optics was still in its infancy.

So for Young Newton to hear just a few words from Xu Yun and, in less than half a minute, realize that the seven colors of light might behave differently in different media was an astounding leap beyond the curriculum.

To be sure,

Young Newton did not yet know that light itself was made of bosons and needed no fermions to propagate—in this era, the most advanced theory of refraction was a mathematical equivalent derived by Snell, and no one knew of it before his death.

It would not be made public as the famous Snell's law until 1678, when Huygens and others examined his manuscript.

Before then, only Descartes had derived an equivalent formula in his Optics.

And there was also a murky matter here whose truth no one could pin down, leaving them still sharing the credit for discovering the law of refraction.

So although Little Ox's explanation had inadvertently included the possibility that visible light could not travel through certain "environments," that was entirely understandable. Frankly, just thinking of the first part was already incredible, wasn't it?

Then, gazing at the small cluster of rainbow-colored light on the ground, Little Ox suddenly thought of something else.

After pondering for a few seconds, he asked Xu Yun:

"Fat Fish, if sunlight can be dispersed into seven colored beams, are they actually different kinds of light?

Also, in theory, those seven beams should be able to recombine into a single beam of white light, right?"

Xu Yun's mouth twitched. Great, he'd gone and brought up two more killer questions.

But by now, he had gradually grown accustomed to the extraordinary thinking of this revered forefather, so he quickly composed himself and replied:

"Sorry, Mr. Newton, I only know the answer to the second question. Given the right conditions, the seven-colored light can recombine into a single beam of white light."

Little Ox was already getting excited. His eyes darted around.

"Fat Fish, didn't you say just now that a prism could also demonstrate dispersion?"

"That's right, and a prism works much better than a lens."

Little Ox nodded slightly, but his brow soon furrowed again.

"A prism? Those are easy enough to find in London. You can buy one at any optical shop on the street, but here in Woolsthorpe..."

Watching Little Ox's tightly furrowed brow, Xu Yun pulled a prism from his pocket as if performing a magic trick.

"Mr. Newton, take a look at this."

Little Ox took it and examined it.

"Eh? How did you get a prism?"

Xu Yun smiled without answering.

Decorative prism pendants were incredibly cheap in the future. Lots of people bought them to hang from their keys or wear as necklaces. Last semester, Xu Yun had taken an optics elective and picked up a prism pendant, which he'd strung onto his key ring.

More than ten minutes later,

a table about a meter in diameter, a prism, and a black wooden board had been placed beside the water tank.

Xu Yun stood the prism upright, skillfully adjusted its angle, then gave Little Ox the OK sign.

A moment later,

a long band of light appeared on the board behind the prism, and it was noticeably clearer than the previous one.

Seeing this, Xu Yun stepped aside and deliberately said nothing, wanting to see how much this younger version of the revered forefather could figure out without any hints.

Little Ox paced over to the table, bent down, and examined the spectrum closely.

Then, after hesitating for a few seconds, he picked up a sheet of black paper.

He cut a circular hole in it and held it outside the prism, on the side facing the light source.

The sheet blocked most of the sunlight. Only a circular beam passed through the hole, and then...

it still formed a long band of light.

At this, Little Ox let out a soft "Huh."

Perhaps because he wanted to talk it over with someone, he suddenly turned to Xu Yun again.

"Fat Fish, have you heard of Mr. Descartes's theory?"

Xu Yun nodded.

"Of course. I even visited the University of Poitiers once.

"Mr. Descartes believed that the color of light came from the medium between the light source and the human eye, and had nothing to do with the source itself.

"He thought color wasn't an inherent property of light, and he also proposed the theory of light-path transformation."

"Well said. But look here."

Holding the sheet of paper in one hand, Little Ox pointed at the long spectrum with the other.

"According to Mr. Snell's equivalent formula and Mr. Descartes's theory, a circular beam passing through a prism should form a circular or elliptical spot.

"But after dispersion, the seven colors form a long spectrum.

"Could it be...

"that Mr. Descartes's theory is flawed?"

After saying this, Little Ox thought for a moment. Before Xu Yun could answer, he picked up the paper and scissors again and made an even smaller hole.

He placed this sheet behind the first prism. That way, the hole let him isolate any beam he wanted from the colored band. (Little Ox drew this apparatus by hand; DOI 10.1098/rsta.2014.0213. It's his own drawing. If you're interested, take a look—it really is the work of a born artist.)

Then Little Ox beckoned Xu Yun closer.

"Fat Fish, I'll call out the measurements, and you record them."

Xu Yun's pupils contracted slightly. He knew Little Ox was swimming step by step toward the "net" he'd ultimately cast, but his expression remained unchanged.

"Of course, Mr. Newton."

The two of them took up paper and pen and began measuring the angles.

"Red light, angle of incidence i = 60°, angle of deviation β = 32.2°."

"Orange light, angle of incidence i = 60°, angle of deviation β = 37.4°."

"Angle of incidence i = 60°, angle of deviation β = 38.7°."

Twenty minutes later, four sets of data—a total of twenty-eight measurements—had been recorded.

The deeper reasons why different kinds of light had different refractive indices in optical glass involved the relative permeability μr and relative permittivity εr. Those constants had to be calculated using Maxwell's equations in the medium. Then one had to establish an intuitive model of the interaction between matter and light, derive the equations of motion using linear damping forces, and solve for the steady state using complex numbers, among other things.

But given that the book hadn't been released yet and it wasn't a good time to psychologically manipulate readers—cough—because the material was too complicated, they only needed to understand the basic conclusions.

After all, there weren't any Maxwell's equations in Little Ox's era, were there?

"Violet 1.532, blue 1.528, green 1.519, yellow 1.517, orange 1.514, red 1.513."

Looking at the sets of figures in front of him, Little Ox couldn't help drawing a deep breath.

It was obvious.

Different colors of light had different, consistent refractive indices. The properties of the seven colors were different.

From this, he could draw a conclusion:

White light was indeed not a pure light. It was made up of different kinds of light.

And that meant...

he might have moved one step closer to the truth of the world.

At the same time,

as Little Ox looked at the light split into seven colors, then joined back into one, something suddenly occurred to him.

His chest rose and fell sharply a few times, and he hurried back into the house.