Newton's Rainbow

Newton's Rainbow

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The nature of light has always fascinated scientists.

Newton, the most creative and complete scientist that History has given, did not escape that fascination. In 1667 he presented his experiment on the decomposition of sunlight to the Royal Society.

At that time, Descartes's idea that light was composed of small corpuscles dominated. The colors were the mixture of light and darkness, in different proportions. Before Newton, Descartes already tried to break down the light, but only managed to get the colors red and blue.

Newton used a pair of glass prisms that, at the time, were popular as children's toys. So it was a very cheap experiment. He prepared a stay in total darkness. Only through a hole in the window did a ray of sunlight enter. He placed the prism in front of the beam of light, so that it would pass through and reflect the light on the opposite wall, 7 meters away. On the wall the colors of the rainbow appeared in an elongated shape, one above the other.

There were two possibilities. Either the prism gave color to the light, or the light was the mixture of all colors and the prism was limited to decomposing it. To verify it, he used the second prism. After the light decomposed in colors placed another screen with a hole, about 3 meters. Through this hole he went through the colors one by one. So, behind the screen, only the chosen color could be seen. For example, red. Once a color was isolated, it made it pass through the second prism and reflected it on another wall.

He found that now he only changed the angle, but not the color. That is to say, if we had isolated the red one, when crossing the second prism it continued leaving red. And so with all colors. He deduced that the colors of the rainbow were pure colors, while the white light was the mixture of all of them. The prism did not add any quality to the light, but rather decomposed it. When projecting the colors and putting them together again, the light was white again.

The experiment caused a sensation in the Royal Society. Newton managed to break down sunlight, but still did not know why. Then the wave nature of light was not yet known.

Today we do know it: colors are waves, and each color has a different wavelength. The wavelengths are longer the closer they get to red, and shorter to violet. When white light enters the prism, each color takes a different path and passes through it at different speeds. They leave reflected with different angle. That is why the colors are separated and are seen on the wall elongated one over the other, and not circular.

It is the same process that forms the rainbow. Water drops act like the prism. Sunlight is reflected on the inner face of raindrops and decomposes in colors.

A curiosity: Newton established that white light broke down into seven colors. I could have said six or another figure (does anyone know what kind of color the indigo is?). It is due to the weight of Greek tradition in our European culture, where number seven is fundamental. Hence it is repeated so much, even in children's stories of European tradition. It is the same reason that led Pythagoras to set seven musical notes.

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