Blue is one of the three primary colors of the traditional color theory (RYB) color model and the additive color (RGB) color model. It lies between violet and cyan in the visible light spectrum. The eye perceives blue when looking at light whose dominant wavelength is between 450 and 495 nanometers. Most blues contain a slight blend of other colors; Azure contains green, while ultramarine contains violet. Clear daytime skies and deep seas appear blue due to an optical effect known as Rayleigh scattering. An optical effect called the Tyndall effect explains blue eyes. Distant objects appear bluer due to another optical effect called aerial perspective. Surveys in the US and Europe show that blue is the color most commonly associated with harmony, fidelity, trust, detachment, infinity, imagination, coolness and sometimes sadness. It is the most popular color in American and European opinion polls, being chosen as the favorite color by nearly half of both men and women. The same surveys also showed that blue was the color most associated with masculinity, just ahead of black, and also the color most associated with intelligence, knowledge, calmness, and concentration.