{"id":120,"date":"2014-08-29T16:25:41","date_gmt":"2014-08-29T16:25:41","guid":{"rendered":"http:\/\/www.beanthinking.org\/?page_id=120"},"modified":"2014-10-27T12:32:07","modified_gmt":"2014-10-27T12:32:07","slug":"blue-sky-thinking","status":"publish","type":"page","link":"https:\/\/www.beanthinking.org\/?page_id=120","title":{"rendered":"Blue sky thinking"},"content":{"rendered":"

\"Blue<\/a>The colour of the sky is often explained with reference to the fact that the air (or the atmosphere) scatters blue light more strongly than it scatters red. \u00a0Consequently, if you do not look directly at the Sun, the sky will appear blue. \u00a0At sunrise (or sunset)\u00a0the light from the Sun has to travel through more of the atmosphere to reach us than it does when the Sun is higher in the sky. The blue light gets ‘scattered out’ at these times, leaving the red that gives us the beautiful sunsets and sunrises.<\/p>\n

While this is true, doesn’t it open up more questions? \u00a0What exactly<\/em> is scattering the light? \u00a0If blue light is scattered more than red, why<\/em> is violet light not scattered more than blue (after all, the order of the rainbow goes from red through blue to violet, why not with the scattering from the sky)? Was the musician Prince onto something with his song Purple Rain? \u00a0Most importantly, what has this got to do with coffee?<\/p>\n

\"John<\/a>
John Tyndall, 1820-1893<\/figcaption><\/figure>\n

These questions are more easily addressed in reverse. \u00a0It is the scattering of light by\u00a0molecules in the air that causes the sky to appear blue but this scattering also occurs for small particles held in suspension in a clear fluid. \u00a0First investigated by John Tyndall (1820-1893), we can explore the effect with milk diluted with\u00a0water as is done in the video below. \u00a0As it concerns particles being held in suspension, the effect should also\u00a0occur in coffee, but for coffee other effects such as chemical absorption of light dominate, making the coffee appear that delightful blackish-brown colour rather than shades of blue.<\/p>\n