why dose rainbow have 7 colours?
Actually a rainbow contains the entire visible light spectrum. Our words and terms for individual colors is (somewhat) arbitrary. There is nothing inherent in physics of color or our eyes why, for instance, light with a wavelength around 470 nanometers should be a distinct color (in this case blue). It seems like a strange idea, because our use of language cements the notion that these colors are somehow special when compared to light at intermediate wavelengths. Tests have been done with children of different cultures to see if there is some hardwired circuitry in the human brain to identify distinct colors the same way. In other words, we see red, blue, green, but does another culture see (what we call) purple, blue-green, and yellow-orange as their primary colors? As an aside it is interesting to note that ancient Greek and Roman texts, for instance, tend to have a lot fewer color terms than modern works of literature. An early test in the 19th century where anthropologists did the
