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by electrostatic and magnetic measurements. Polarization experiments 

 confirm that light waves are transverse. Optically determined values 

 of the velocity of light, c, agree with those predicted for electromagnetic 

 waves to better than one part in a million. 



Further evidence that light consists of electromagnetic waves is its 

 continuity with radiation produced by other methods. Using tech- 

 niques which overlap at their wavelength limits, one may produce 

 radio waves, microwaves (radar), heat waves (infrared), light waves, 

 ultraviolet rays, X rays, and y rays. Thus, all of these are part of the 

 same basic phenomenon: electromagnetic waves. No explicit use will 

 be made of the electromagnetic properties of light waves in the chapters 

 on the eye or on vision in this text. 



C. Light as Photons 



The electromagnetic wave theory correctly describes the transmission of 

 light, but a number of other effects are impossible to understand 

 without the quantum theory. These include the characteristic spectra 

 of atoms, the absorption spectra of atoms and molecules, the photo- 

 electric effect, black-body radiation, and the failure of the equipartition of 

 energy for electrons in a metal and for the vibrations of diatomic gases 

 at room temperatures. All of these and many other phenomena have 

 been explained only in the terms of quantum mechanics. Quantum 

 mechanics teaches that energy comes in packets or quanta. The 

 probability of finding the packet at a given place is determined by the 

 square of the amplitude of a wave function. In particular, for electro- 

 magnetic waves, the quantum theory states that energy E comes in 

 photons each having the energy 



he 



E = j (5) 



where h is Planck's constant, which is about 6.6 x 10 ~ 27 erg -sec, c is 

 the velocity of light, and A is the wavelength. The relative probability 

 of finding a photon at a given place is essentially identical to the intensity 

 computed on the basis of the electromagnetic wave theory. Strictly 

 speaking, a measured quantity has been specified by a probability, but 

 experimentally these two are indistinguishable. 



The photon nature of light is important in describing the threshold 

 of vision. It is likewise necessary, in Chapter 19, where vision is dis- 

 cussed on the molecular level. In the latter case one may ask: How 

 many photons react with a molecule; how do the photons change the 

 sensitive molecules; and how are the resulting small bursts of energy 

 transduced to neural impulses ? Unfortunately, it will appear that one 

 cannot give a complete answer. Nonetheless, the language of photons 



