Why doesn't the outside world appear blue even though so much light comes from the blue sky?
Category: Earth Science Published: February 10, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Actually, a lot of the outside world on a sunny day is tinted blue because of the blue sky. The blue tinting of the outside world is quite striking if you know how to look for it.
Sunlight that has just left the sun contains a relatively equal mix of all spectral colors and is therefore white. When the sunlight passes through the air in earth's atmosphere, some of the light is scattered sideways by the air molecules. The air molecules scatter a little bit of all colors out of the forward-traveling beam of sunlight, but blue and violet colors are scattered the most, giving the sky a whitish-blue appearance. Since more blue and violet light have been removed from the forward traveling beam than other colors, the direct sunlight that reaches earth's surface is slightly tinted orange-red compared to the sunlight given off at the sun's surface. But this orange-red tint is so small (because the total amount of light scattered out by the atmosphere is small compared to the sunlight that continues on in the forward direction) that direct sunlight that reaches earth's surface is still white.
The white direct sunlight is much brighter than the blue skylight, and this is the main reason that the entire outside world is not completely blue. White light contains all colors, so a scene illuminated by white light will show all the innate colors of the objects in the scene. When illuminated by white light such as sunlight, we are able to see the grass as green, the flowers as red and wood as brown. The world shows its usual mix of colors when illuminated by white light.
White sunlight is so bright compared to blue skylight that when an object is in direct sunlight, it just looks like its innate color. But, when an object is in shadow so that direct sunlight cannot reach it (and there are not too many clouds that block the skylight), the object is now being illuminated only by blue skylight. Shadows on a clear day are therefore blue tinted. The more shadows there are, the more the world around you is tinted blue. When the sun is low in the sky and behind the trees, the entire landscape can end up being shadowed from direct sunlight and therefore be blue tinted. We often don't notice the blueness of the outside world because our human eyes are better at seeing relative color than absolute color. This means that our eyes are better at picking out color differences than at picking out an overall color on an absolute scale, as shown in the image above. But the blue tint of the outside world becomes obvious if you look into an area of deep shadow on a bright, clear day. The blue tint of shadows can make for dazzling scenery worthy of being painted or photographed.
