What Is Light?

Imagine being in the park and looking at the tree branch. We know that light is repelled from the leaf in your eye, and you know that it is green – exactly what is light?

Two early ideas date back to the 17th century: English scientist Isaac Newton thought the light was composed of small particles (called corpuskule) emitting hot objects (like the sun or the fire) while his contemporaries, the Dutch physicist Christian Huygens, considered light a kind of wave vibrating up and down as it moves forward.

But neither had the concept of light. (Newton did not know what his corpusculus was made of, Huygen had no concept of moving as a wave, but the question of whether the particle or wave photon is still not completely resolved.)

The first steps towards understanding the composition of light are followed up to Copenhagen in 1820 when Danish scientist Hans Christian Ørsted delivered a lecture on electricity

In addition to the battery he used in his demonstration, he accidently found a compass. Ørsted noticed a sudden spin of a magnetic needle when he turned on or turned off the battery. This meant that electricity and magnetism were connected – or, as it is later described more formally, a variable electric field creates a magnetic field.

Then, 11 years later, an English scientist, Michael Faraday, found that the opposite is true: a changeable magnetic field also creates an electric field.

Scottish physicist James Clerk Maxwell collected all the ideas of electricity and magnetism (and a few ideas from the side) and linked them to a connected theory of "electromagnetism."

But Maxwell's most important discovery was when he combined the work of Ørsted and Faraday to describe the essence of light.

He realized that a variable electric field could create a variable magnetic field, which would then create another electric field and so on. The result would be a self-sustaining electromagnetic field that is infinitely repeating and traveling incredibly fast

How fast? Maxwell managed to calculate this, as well, about 300,000,000 feet per second – quite close to what was recently measured as the speed of light.

And it is light: an electric field linked to a magnetic field that flies through the universe.

You can imagine these two fields as dance partners, interwoven with eternal embrace. In order to continue self-generating, the electrical and magnetic components must keep up. Tango needs two.

Now we know there is a whole spectrum of electromagnetic waves that differ in wavelengths. (The wavelengths in this image would be the length of the dance steps.)

At the short end of the spectrum, high-energy gamma rays may have wavelengths significantly lower than hydrogen atoms, while at a longer end, low-energy radio waves can be as long as the Jupiter planet is wide. Visible light is a very thin slice of the electromagnetic spectrum, wavelengths of about 400 to 700 nanometers, which is approximately the width of the bacterium E. coli or about 1% hair hair width

You may wonder why we see this light range rather than other wavelengths. There are two main reasons

First, "vision" usually involves a kind of chemical reaction that is caused by light. Carbon-based chemistry in our cells sparked light around the visible range. Longer wavelengths do not carry enough energy to trigger reactions, and light shorter wavelengths carry too much energy and can damage the delicate chemistry of life (for the same reason, for example, ultraviolet radiation causes burns).

Secondly, the range of 400-700 nanometers can travel quite far in the water before it is absorbed (so the glass of water seems transparent to us – almost all visible light goes through it). The first eyes evolved under the sea, so this range of light had the greatest evolutionary advantage compared to other wavelengths.

Now, back to the park. When you look at the leaf, the light that enters your eye is the wave of electrification and magnetism of a certain wavelength. Light strikes your retina and triggers a certain form of chemical change in your cone cells, which your brain recognizes as "green."

The next time you meet someone with nice eyes, you can tell them they have beautiful detectors of electromagnetic waves.

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