Rupali and I spent a wonderful weekend in the Smoky Mountains over this Thanksgiving break. We stayed at the Berry Springs Lodge, a quaint little bed & breakfast with a breathtaking view in the middle of the hills. It is run by a family and Patrick and Sue were the most welcoming of hosts.
Amongst activities like mountain biking, croquet, fishing and plain old relaxing, we thoroughly enjoyed waking up at seven in the morning to catch the sunrise. The rolling hills and the thinly clouded skies provided an excellent backdrop for the beautiful silhouettes and deep hues of red. I am not too much of a morning person and I don’t remember waking up for too many sunrises but this one was special.
While driving back from the Smokies, Rupali and I got into a discussion of why the sunsets were such a brilliant red and why the sky that we see is almost always blue. The answer is fairly simple and lies in the phenomenon of Rayleigh scattering.
Simply put, light from the sun travels through the atmosphere and is scattered by the suspended dust/smog/clouds as well as the molecules of air. The color of the sky is a result of the scattered light seen by the observer in a direction away from the sun. The intensity of light scattered is inversely proportional to the wavelength of the scattered light and since blue light lies towards the lower wavelength end of the spectrum, it is scattered more strongly than red light. This results in what we see as a blue sky.
During sunsets and sunrises we look towards the sun and see not only scattered light but light coming directly from the sun. In such a situation, since blue light is scattered the most as the sunlight passes through the atmosphere, only the red end of the spectrum reaches our eyes. This cartoon has a very simple and clear illustration of this effect.
If there was no atmosphere to scatter light, we would observe a dark sky which is exactly what is observed from space. The sun looks like a white ball and the sky is pitch black dotted with wondrous stars.
Yahoo will help you learn more and the Exploratorium has some fun experiments you can do with children to help them understand light scattering. If you have access you can also pick up this fine book and read more about all sorts of scattering: Light Scattering by Small Particles - H. C. Van De Hulst.
The key difference to remember is that blue skies are caused by scattered light whereas red sunrises/sunsets are caused by transmitted light.