Why Rainbow has seven colours only ?

We usually say there are seven colours in a rainbow but this is a bit of an understatement. A rainbow is an arched spectrum of light that we see in the sky when the Sun shines onto raindrops. It has thousands of different colours. Without getting too heavily into science, let's just look at some basic things about colour. We might remember being taught at school that there are three primary colours (red, yellow and blue), and mixing these, we can get the three secondary colours (red + yellow = orange, yellow + blue = green, and blue + red = violet). Each colour has a wavelength, as we see in the chart on the right. Beyond the red 780 nanometers (nm) mark, we have infrared, which we cannot normally see with the unaided eye. And beyond the violet 390 nm, we have ultraviolet, which again, is outside our normal vision. Mixing the primary and secondary colours gives us more colours (for example red + orange = salmon) but the wavelengths of these tertiary colours are not nearly as wide, and therefore not so visible to the eye, as the six cardinal colours. So there we have it. Only six main colours in the rainbow. So what is the seventh colour? The answer is of course indigo, a tertiary mixture of blue and its neighbouring colour violet. Indigo has a very narrow band (around 450-440 nm), and as such, has very little reason for being included in our description of the rainbow. So why is indigo there? Well, when astronomer, natural philosopher and Unitarianist Isaac Newton divided up the visible spectrum, he decided there should be seven colours to link them with the seven 'planets' known at that time. This was one of his missions in life: to show an integrated and harmonious cosmos. Indigo is a rather drab mixture of blue and violet. Maybe a nice salmon colour, mixing red and orange, could have been chosen. But Newton was keen to show the relationship between colour and music. He felt that it was no coincidence that the musical diatonic scale had seven notes (do-re-mi-fa-so-la-ti), although he seems to have ignored the important difference that the musical scale is cyclic (i.e. it repeats at each octave) whereas the colour spectrum is not. Nevertheless, he decided that red corresponded to D on the musical scale and therefore indigo became B (going down the colours and up the scale). Did the frequency of that note give Newton a 1931 Duke Ellington style Mood Indigo http://www.seiyaku.com/customs/crosses/rainbow.html

The rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of about 40°–42°. This angle is independent of the size of the drop, but does depend on its refractive index. As seawater has a higher refractive index than rain water, the radius of a 'rain'bow in a sea spray is smaller than a true rainbow. This is visible to the naked eye by a misalignment of these bows[1]. Since the water is dispersive, the amount that the sunlight is bent depends upon the wavelength, and hence colour, of the light's constituent parts. Blue light is refracted at a greater angle than red light, but because the area of the back of the droplet has a focal point inside the droplet, the spectrum crosses itself, and therefore the red light appears higher in the sky, and forms the outer colour of the rainbow. Contrary to popular belief, the light at the back of the raindrop does not undergo total internal reflection; however, light that emerges from the back of the raindrop does not create a rainbow between the observer and the Sun. The spectra emitted from the back of the raindrop do not have a maximum of intensity, as the other visible rainbows do, and thus the colours blend together and do not form a rainbow.

Well, according to Terry Pratchet there is 8 colours ;) Maybe there are more colours. We ar limited in what our eyes can handle, i.e. we cant see UV or IR..

Those seven are what are called "primary colours" . light through rain drops creates the rainbow. Light through a prism will demonstrate the primary colours.

I am going to take a guess at this. Okay if you take the three primary colors and mix them you will get the other colors that follow in the rainbow. I don't know I am taking a guess at this. Does this make any sens e to you all?

For many years, people believed that a spectrum (also called a rainbow) has seven colors. Technically, a rainbow contains millions of colors. Our eyes only pick out colors that are easy to see. Millions of colors are made as all the colors blend together. The six main colors seen in a spectrum and a rainbow are (in order) red, orange, yellow, green, blue, and violet. Indigo used to be included as a color between blue and violet.

Sunlight split the raindrops into many many colours and shades of colours, that is why some seem brighter than others. Our eyes can only see a fixed range which is sad. I have crystals in my lounge windows so that I see rainbows everyday if the sun comes out to play. They are a small miracle we can all enjoy.

Has to do with the color spectrum of light and what the eye can actually see.

I guess it wouldn't be a rainbow if it has less than 7 or more than 7 colors. greetings. : )

GOd knows

VIBGYOR.

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God knows