Question

why soectrum is giving only 7 colours and 7 rays

Answer 1

In the 17th century, he was the one who realized that, when we break white light apart using a prism (or rain drops), we get the visual spectrum of colored light (otherwise known as the rainbow).



As you can see, in the visual spectrum, each color bleeds into its neighbors. It’s not a distinct set of colors but rather, well, a spectrum4. But Newton decided we should probably break this spectrum up into chunks, so we could more easily talk about it. But how many divisions should there be…?

Ancient Greece and the magic of 7
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Via Reuters

Seven is lucky. Or so those of us in Western Cultures have always been told. But why? We can trace the roots of this association back to the 6th century BC and a dude named Pythagoras5. Now, Pythagoras loved numbers. And he loved applying numbers to real-world phenomena. He’s credited with discovering the fact that musical notes (of which there are seven) could be translated into mathematical equations, and he had a theory about how the heavenly bodies (seven of which where known about at the time) moved according to mathematical patterns.

Noticing a pattern? Pythagoras did: his observations showed that 7 was a magical number that somehow connected disparate phenomena. He further saw it as the sum of the spiritual (3) and the material (4).

Pythagoras also started a school, and the ideas he espoused grew into a philosophy called Pythagoreanism, based on mathematics and mysticism. Pythagoreanism influenced some of the most well-known classical thinkers, including Aristotle and Plato.

And thus we now have seven days of the week, seven liberal arts subjects, seven deadly sins, seven wonders of the world and seven dwarfs.

So how did it get applied to the rainbow?

The circular route from Pythagaros to Newton
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Newton not only associated color with music, but he postulated that the color spectrum was cyclical, like octaves. Via Wikipedia.

The Pythagorean philosopher Philoalaus is the first known person to posit that the earth resolved around a “central fire” (rather than everything revolving around the earth). This theory was in turn used by Copernicus, who is widely credited with developing the heliocentric theory of planetary motion. And Newton relied on Copernicus’ work when developinghis own theory of gravity.

So TL;DR Newton thought the Pythagoreans were pretty great.

When he started his work with color, he originally only subdivided the spectrum into five colors (red, yellow, green, blue and purple), but revised the number to seven, adding orange and indigo, because Pythagoras believed that there was a connection between color and music. And there are seven natural notes, so there should also be seven principal colors.

Math, music, numerology and a couple of dead guys. That, kids, is why there are seven colors in the rainbow.

Answer 2

Because it is meant to be !