Irish physicist John Tyndall discovered that sunlight is scattered by molecules in the earths atmosphere.
Now known as the ‘Tyndall Effect’, the shorter the lights’ wavelengths, the more strongly it is reflected and scattered by these molecules. Colour receptors in the retina of the human eye are most responsive to wavelengths of red, green and blue light.
The wavelengths of blue light are shorter than those of both red and green light. Therefore, the sky appears blue to the naked eye!