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Free Will 319 The Observer I am looking out of my office window. It is a sunny autumn day and I have a beautiful view over London, but if I squint a little I can also see my reflection. The window in front of me is not perfect. Although it is mostly transparent, the glass also reflects some of the light. If you think of light as particles, the majority of the photons go through. But some bounce back. I’m going to show you that the behavior of these photons is governed by the observer — me! The laws governing light, and most of the strange and wonderful effects it has, were first stated by Isaac Newton. Newton was an extraordinary man. He discovered many of the physical principles we use today, and his view of the Universe reigned unchallenged until Einstein’s discovery of relativity. He was also, by many accounts, a nasty piece of work. Not only was he a famous academic, he also head of the Royal Mint. He is said to have taken great pleasure in having forgers hanged on Tower Hill. He claimed the invention of differential calculus despite it being invented independently by Gottfried Leibniz. Newton managed to have himself appointed to chair the committee reviewing Leibniz’s work and determine who had come up with the idea first. Unsurprisingly, the committee found for Newton! We see Newton's laws of reflection and transmission in all manner of everyday products, for example, the antireflective coatings of camera lenses or the screen of your smartphone. Manufacturers cover the glass in these products with coatings just a few molecules thick. Interference between the layers kills the reflections. On a very expensive lens several different layers are used; some kill red light and others kill blue light. Together they suppress most of the reflection. If it were not for these coatings you would be unable to go to the park on a summer’s day and read your iPad. We need to think about reflection and transmission to demonstrate our role as observers. A windowpane has two surfaces. Both surfaces reflect light, and if you look closely you will see your face is really reflected twice. You might think this is simply a double reflection but this is not so. Light behaves like waves. As with water waves, they interfere with one another. If two light waves are at the top of a crest as they meet, the result is a crest of double height. If both are at the bottom, you have a double trough, and if one is a crest and the other a trough you get nothing as they cancel each other out. You can see this effect in waves on the surface of a pond. When light strikes a window pane, the light has two chances to reflect: one from the front surface and the second from the back surface. These two reflections interfere with each other. And, again, when you HOUSE_OVERSIGHT_016009