North light (in the Northern Hemisphere) is sunlight coming through a north-facing window. Because it does not come directly from the sun, it remains at a consistent angle and colour throughout the day and does not create sharp shadows. It is also cooler than direct sunlight due to the way the Earth's atmosphere scatters light via Rayleigh scattering. These properties make it the natural light of choice in certain styles of architecture, painting and photography. In addition, the cool colour of north light has been studied for its effect on our perception of art in galleries and museums. South of the equator (in the Southern Hemisphere), the same characteristics are seen in south light. Formation and Properties Because the sun passes to the south of most observers in the northern hemisphere, north light is the light coming from the sky, rather than directly from the sun. This is the reason for its diffused nature, as well as why it casts softer shadows than direct sunlight and remains more consistent in colour than light from the east or west (which would be affected by sunrise and sunset respectively). The colour of diffused daylight is 5500 - 6500K, meaning that north light is cooler (more blue tinted) than light from other directions. The main cause of this is Rayleigh scattering, which was first mathematically described by British physicist Lord Rayleigh (John William Strutt) in 1871. Earth's atmosphere contains large proportions of nitrogen and oxygen which, via Rayleigh scattering, scatter short wavelengths of electromagnetic radiation more efficiently than longer wavelengths. Thus, light scattered or diffused away from the main beam of sunlight appears cooler while direct light (especially during sunlight or sunset) appears warmer. North light is also less bright than direct sunlight, as only a portion of incoming white light is scattered. The brightness of north light can be between 10,000 and 30,000 lux, depending on location and season, while direct sunlight can be as bright as 100,000 lux. In Architecture The use of natural light in architecture is called daylighting. It first rose to professional importance during the Roman Empire, when architects struggled to improve the ambience of public and religious spaces while reducing glare. At first, this involved physical structures such as clerestory windows and roof slits, however by the first century CE the direction of light began to play a bigger role. The Pantheon, rebuilt during the reign of