How Did We Figure Out What Light Is?

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If a light is on and no one is around to see it, is anything actually lit up?

The ancient Greeks didn’t think so.

They thought vision was due to raysshot outand returned to the eye.

That’s… not how it works.

Unless your name is Cyclops.

It took over 2,000 years, from those Greeks all the way to Einstein, to figure out how

we see the world….

And each answer led to other questions - Is light a wave, or a particle?

Is it instantaneous, or does it travel at some speed?

Does it even exist independent of our perception?

Through these brilliant discoveries about light, weve gradually illuminated our place

in the universe.

[INTRO]

Beyond what we can touch, smell, hear, and

taste, we experience the universe through light.

The first theories of vision came from the Greeks, who thought our eyes worked more

like flashlights than cameras.

It made sense: at night, animals seemed to have 'fire in their eyes' - so why not humans

too?

Early mathematicians like Euclid worked out that light mostly traveled in straight lines,

and deciphered the geometry of how it’s bent and reflected.

We still use a lot of that today, but the nature of sight changed in the 10th century.

An Arab Muslim named Ibn Al-Haytham’s had a revolutionary idea.

Instead of just accepting the Greeksideas, he actually tested them.

He set up specific situations, varied different conditions.

He did experiments, making him perhaps the first true scientist.

Al-Haytham set up two lanterns outside a dark room.

There was a hole in the wall.

Inside the room, he saw two spots of light.

Cover a lantern?

Its spot disappeared.

This reversed Euclid’s idea.

Our eyes don’t give off light, theyre just receivers.

It proved that light existed outside our bodies.

Yeah, I know, it’s obvious, but this was a HUGE step.

But there was still a bunch we didn’t understand

For most of our life experiences, light moves so fast, it might as well be instantaneous.

For a long time this is what everyone thought.

Dutch astronomer Ole Romer noticed eclipses of the moon Io came later than expected when

Earth and Jupiter’s orbits were moving away, and sooner than expected when the planets

moved closer.

This difference was explained by the time lag for light to make up the distance.

Light travels at a speed.

Once it was understood that light was a thing that traveled from one place to another, people

like Newton began to take it apart.

Prisms separate light and recombine it because different colors are all bent by different

amounts, proving color is an intrinsic property of light, not just a trick of our perception.

Newton noticed shining colored light on objects can make them appear any hue.

But if you use theirtruecolor, they shine most brightly.

This showed Newton pigments in objects workedsubtractively’ - white light contains

every color, and objects would absorb the colors they weren’t, and reflect the colors

we see.

Newton’s contemporaries, found that the bending of light in prisms, refraction, could

be explained if light was actually a wave, and if that wave had different speeds in different

media.

And the famous double slit experiment found interference of light in a wave-like fashion,

like ripples in a pond.

Despite all this evidence that light acted like a wave, Newton and many others held on

to thecorpuscular theory” - that light is composed of many many tiny balls, all with

their own color.

Newton was also really into alchemy, which had a thing for the number 7, so he divided

the rainbow into 7 bands.

Newton also thought color should be cyclical, so he mapped it to the musical scales.

Of course today, we know light comes in a spectrum, not a circle, but to get that, we

have to go forward another hundred years, to the astronomer William Herschel.

Like Newton, Herschel used a prism to break up sunlight into its colors to measure temperatures

across the rainbow.

But when he put a thermometer in the dark, just past the red, it registered the highest

temperature of all!

There must be invisible light, beyond what we can see.

He called theseheat rays,” but today we know them as infrared.

Johann Ritter decided to test the other end of the spectrum, past the purple.

There, he found that chemical reactions happened fastest.

He called theseoxidizing rays,’ but today we call that higher energy light ultraviolet.

In the 19th century, James Clerk Maxwell finally unified electricity and magnetism.

He calculated that the speed ofelectromagnetic waveswas the same as the speed of light.

The particle theory was dead.

Electromagnetic waves *are* light!

Heat raysandchemical raysand visible light were all part of the same beast,

the electromagnetic spectrum, a continuous gradient of waves, all made of the samestuff”,

extending out beyond each end of the tiny sliver we can see.

At one end, long waves of "invisible light" were created using electric currents and antennas.

Radio waves and all wireless communication, were born.

At the other end, we discovered invisible light with short waves and even more powerful

energies: X-rays and gamma rays.

The finishing touches came 100 years ago.

Max Planck developed a theory of how energy is spread across the spectrum, in little bits

he called quanta, but to make it work, light had to be a particle, like Newton thought,

each with specific energy.

Einstein realized this could explain the photoelectric effect . If light behaved likepackets

of waves, what we now call photons, it could explain why shining light beams on metals

could release electrons.

Single photons can knock single electrons, out of their atom.

By combining these features they resolved the debate.

Llight is neither a wave nor a particle, but bothsimultaneously.

And thiswave packet,’ idea set the stage for modern quantum theory.

But that’s a story for another day.

From humble beginnings in Greece and the Arab world, over two thousand years weve

learned a lot about how we see the world, and made the universe bigger and smaller in a lot of ways.

Stay curious.