Back-of-the-Envelope Physics by Clifford Swartz

By Clifford Swartz

Physicists use "back-of-the-envelope" estimates to ascertain even if an idea may very well be correct. in lots of situations, the approximate answer is all that's wanted. This compilation of one hundred and one examples of back-of-the-envelope calculations celebrates a quantitative method of fixing physics difficulties. Drawing on a life of physics examine and approximately 3 many years because the editor of The Physics instructor, Clifford Swartz presents easy, approximate suggestions to physics difficulties that span a extensive diversity of subject matters. What observe do you get if you happen to blow around the most sensible of a Coke bottle? may well you reduce weight on a vitamin of ice cubes? How can a fakir lie on a mattress of nails with out getting damage? Does draining water within the northern hemisphere fairly swirl in a distinct path than its counterpart lower than the equator? In each one case, just a couple of traces of mathematics and some ordinary constants resolve an issue to inside a number of percentage. overlaying such topics as astronomy, magnetism, optics, sound, warmth, mechanics, waves, and electrical energy, the ebook presents a wealthy resource of fabric for academics and a person drawn to the physics of lifestyle.

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But there can be a lot of processing between the initially received signal and the final image! That is what this book is about. The interaction between the wave and the detector is basically one between the electromagnetic field and the atoms of the device, causing an output electric current which is the received signal. In astronomy, the quantum nature of electromagnetic waves becomes relevant when we consider the detection of light, since the light signals are almost always very weak. Under these conditions, the energy transport appears not to be † The vector notation, invented after Maxwell’s days, makes the derivation seem almost trivial!

The fine periodic fringes are modulated by a rather coarse envelope, resulting from the widths of the individual slits, which can be ignored in the present discussion; it is the fine fringes that we are interested in. You will notice that the farthest lamps, which appear smallest, give the clearest fringes. Looking at closer sources, which appear larger and brighter, the fringes † Young would have had to put salt into a flame to create a monochromatic source. Today we get such sources free of charge, because the most common high-intensity street lights provide approximately monochromatic light at 589 nm (Na) and 546 nm (Hg).

However in very light rain, drops of approximately uniform size fall in uncorrelated positions at random times; we can certainly calculate average probabilities, but we can predict nothing about when or where a particular raindrop will appear. The “shadow” of an umbrella (observed as a dry patch on the ground) will be completely unrecognizable or very fuzzy until some minimum density of raindrops has fallen; the more detail we want about the edge of the umbrella, the more raindrops we need. Images taken with few photons have similar characteristics; their resolution is severely compromised by small photon counts, and so in astronomy every effort has to be made not to lose photons.

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