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Let's do a historical #OpticsLessonOfTheDay on the birthday of my favorite scientist ever, Michael Faraday (1791-1867)! Though he is relatively unknown to the public, he is inarguably one of the greatest scientists who ever lived... and an amazing person, as well. 
By societal standards of the time, Faraday should have lived a mundane, uneventful life. He was born the son of a blacksmith in Surrey in the UK. He was raised with little formal education, and was apprenticed to a bookbinder at age 14.
But working at a bookshop gave Faraday access to lots of books, including science books, and his master George Ribeau was a decent fellow who gave Faraday leisure time to indulge his curiosity.
Okay, let's to an #OpticsLessonOfTheDay: why is the sky blue? The answer is an interesting mix of several different aspect of physics as well as the working of the human eye.
First off: we note that the sun radiates light somewhat uniformly over the entire visible spectrum of light, which ranges from 380 to 750 nanometers. (Image via Wikipedia.) It peaks a bit in the middle of the spectrum, so we picture the sun as a bit more yellow than white.
Now, during the day, we see blue light everywhere *except* in the direction of the sun. This is because the gases in the atmosphere preferentially scatter blue light. All the blue you see is from light scattering off of atmospheric molecules.
So I go to a movie and come out seeing that all sorts of people are explaining the phenomenon wrong. To me. An optical physicist.
So let's have a quick #OpticsLessonOfTheDay: there are a bunch of different ways that you can get rainbows, and they're all different!
The most familiar rainbow effect is the one that Isaac Newton studied using a prism, and also forms the cover of Pink Floyd's Dark Side of the Moon:
Let's start a new series of #OpticsLessonOfTheDay! First up: the three main branches of optical physics! I break things down into geometrical optics, physical optics, and quantum optics.
Geometrical optics is the oldest branch, and consists of treating light as propagating in straight lines except for reflection and refraction at interfaces.
Geometrical optics doesn't really consider any "physics" of light, and both the wave theory and the particle theory of light were argued to explain the geometrical properties of light. Huygens argued wave, Newton argued particle; Newton basically "won" in early 1700s.
