MOTION MOUNTAIN

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118 3 what is light? light Challenge 119 e Challenge 120 s Ref. 73 Ref. 74 Ref. 75 Ref. 76 F I G U R E 72 A commercial light mill turns against the light (Wikimedia). the small dust particles that detach from it. For this reason,the tail always points away fromtheSun, asyoumight wantto checkat thenextopportunity. Today, weknowthat Keplerwasright;butproving the hypothesis is not easy. In order to detect the radiation pressure of light, in 1873, William Crookes* inventedthelightmillradiometer.Thelightmill consistsoffourthinplates,blackononeside andshinyontheother,thataremountedonavertical axis, as shown in Figure 72. However, when Crookes finished building it – it was similar to those sold in shops today – he found, like everybody else, that it turned in the wrong direction, namely with the shiny side towards the light! (Why is it wrong?) You can check it by yourself by shining a laser pointer on to it. The behaviour has been a puzzle for quite some time. Explaining it involves the tiny amount of gas left over in the glass bulb and takes us too far from the path of our adventure. It was only in 1901, with the advent of much better pumps, that the Russian physicist Pyotr Lebedew managed to create a sufficiently good vacuum toallowhimtomeasurethelightpressurewithsuchanimproved, true radiometer. Lebedew also confirmed the predicted value of the light pressure and proved the correctness of Kepler’s hypothesis about comet tails. Today it is even possible to build tiny propellers that start to turn when light shines on to them, in exactly the same way that the wind turns windmills. But light cannot only touch and be touched, it can also grab. In the 1980s, Arthur Ashkin and his research group developed actual optical tweezers that allow one to grab, suspend and move small transparent spheres of 1 to20 µm diameter using laser beams. It is possible to do this through a microscope, so that one can also observe at the same time what is happening. This technique is now routinely used in biological research around the world, and has been used, for example, to measure the force of single muscle fibres, *William Crookes(b.1832London,d.1919London),chemistandphysicist,presidentoftheRoyalSociety, discovererofthallium, andbelieverinspiritualism. Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net
what is light? 119 Light can rotate macroscopic objects: Light can rotate tiny objects, such as carbon nanotubes: suspension wire Ref. 76 circularly polarized light beam F I G U R E 73 Light can rotate objects (© A.C. Ferrari) F I G U R E 74 Umbrellas decompose white light: look at a small lamp through a black umbrella at night (© Wikimedia). by chemically attaching their ends to glass or Teflon spheres and then pulling them apart with such optical tweezers. But that is not all. In the last decade of the twentieth century, several groups even managed to rotate objects, thus realizing actual optical spanners. They are able to rotate particles at will in one direction or the other, by changing the optical properties of the laser beam used to trap the particle. In fact, it does not take much to deduce that if light has linear momentum, circularly polarized light also has angular momentum. In fact, for such a wave the angular Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net
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ChristophSchiller MOTION MOUNTAIN t
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Editiovicesima octava. Proprietassc
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DieMenschenstärken,die Sachenklär
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8 preface PHYSICS: Describing motio
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10 preface Your donation to the cha
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12 contents Do we see what exists?
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Light, Charges and Brains Inourques
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16 1 electricity and fields F I G U
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18 1 electricityandfields F I G U R
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24 1 electricity and fields TA B L
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28 1 electricity and fields Challen
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30 1 electricity and fields If elec
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34 1 electricityandfields F I G U R
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42 1 electricity and fields Oersted
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44 1 electricity and fields Vol. IV
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48 1 electricity and fields Ref. 24
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50 1 electricity and fields Page 21
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52 1 electricity and fields Ref. 29
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64 1 electricity and fields battery
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Page 140 and 141: Chapter 4 IMAGES AND THE EYE - OPTI
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168 4 imagesandthe eye-optics F I G
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172 4 images and the eye - optics C
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176 4 images and the eye - optics X
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Chapter 5 ELECTROMAGNETIC EFFECTS R
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208 5 electromagnetic effects Ref.
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230 6 classical electrodynamics Pag
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236 7 the story of the brain F I G
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238 7 the story of the brain Vol. V
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240 7 the story of the brain TA B L
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242 7 the story of the brain consci
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244 7 the story of the brain Ref. 2
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246 7 the story of the brain F I G
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248 7 the story of the brain Vol. I
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254 7 the story of the brain ∗∗
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Chapter 8 THOUGHT AND LANGUAGE Ref.
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258 8 thought and language Challeng
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260 8 thought and language Vol. VI,
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262 8 thought and language F I G U
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264 8 thought and language Challeng
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266 8 thought and language Ref. 241
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268 8 thought and language Vol. IV,
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270 8 thought and language . . 0 1
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272 8 thought and language Ref. 251
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274 8 thought and language ⊳ Ever
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276 8 thought and language F I G U
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278 9 concepts, lies and patterns o
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Chapter 10 CLASSICAL PHYSICS IN A N
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322 10 classical physics in a nutsh
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328 a units, measurements and const
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342 challenge hints and solutions n
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358 challenge hints and solutions C
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360 challenge hints and solutions V
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362 bibliography 10 Thiseffect hasf
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364 bibliography 37 A discussionof
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366 bibliography M.B. van der Mark
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368 bibliography Vol. I, page 96 19
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370 bibliography S.W. McCuskey, F.C
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372 bibliography 119 A complete lis
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374 bibliography F I G U R E 178 Th
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376 bibliography in unserer Zeit33,
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378 bibliography private communicat
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382 bibliography Vol. IV, page 135
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384 bibliography and 304. 260 For a
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386 bibliography Springer, 1996. wh
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CREDITS Acknowledgements Many peopl
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390 credits tute of Molecular Patho
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NAME INDEX A Abbott A Abbott,T.A.37
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396 name index D Democritus Democri
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398 name index H Holmes Holmes, C.D
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400 name index M Mohr Mohr, P.J. 38
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402 name index S Sheldon Sheldon, E
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404 name index Z Zybin Motion Mount
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406 subject index A units 328 Aplys
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408 subject index C classical class
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410 subject index E Earth Earth age
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412 subject index F finite finite 2
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414 subject index I infrared photog
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416 subject index L Lorentz Lorentz
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418 subject index M motion motion a
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422 subject index S Scarabeus Scara
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424 subject index T table d’Unit
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426 subject index W Wien’s Wien
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