SOME CONrEMPOR.IRy .iDr.iXCIlS IX PHYSICS— Vn 311 



train of plain- waves, flowiiii; against a blackened plate facing normally 

 at;ainst the tlirection in which they advance, which utterly absorbs 

 them. This wave-train shall have an intensity /; by which it is 

 meant, that an amount of energy / apf)ears, in the form of heat, 

 in unit area of the blackened plate in unit time. I'nrtherniore, the 

 radiation is found to exert a pressure p against the blackened plate; 

 by which it is meant, that imit area of the plate (or the framework 

 upholding it) acquires in imit time an amount of momentum p. 

 According to the classical electromagnetic theory, verified by ex- 

 |x?rience, /> is equal to / V. I'nit area of the plate acquires, in unit 

 time, energy to the amount / and momentum to the amount / r. 



Where is this energ>", and where is this momentum, an instant 

 before they appear in the plate? One might say that they did not 

 exist, that they had vanished at the moment when the radiation left 

 its source, not to reappear until it arrived at the plate; but such an 

 answer would be contrary to the spirit of the electromagnetic theory, 

 and we have long been accustomed to think of the energy as existing 

 in the radiation, from the moment of its departure from the source 

 to the moment of its arrival at the receiver; the term "radiant energ\" 

 implies this. Momentum has the same right to be conceived as exist- 

 ing in the radiation, during all the period of its pas.sage from source to 

 receiver. In the system of equations of the classical electromagnetic 

 theor\', the expression for the stream of energy through the electro- 

 magnetic field stands side by side with the expression for the stream of 

 momentimi flowing through the field. If the second expression is not 

 so familiar as the first, and the phra.se "radiant momentum" has not 

 entered into the language of physics together with "radiant energy," 

 the reason can only be that the pressure which light exerts upon a sub- 

 stance is ver>- much less conspicuous than the heat which it communi- 

 cates, and seems correspondingly less important, — which is no valid 

 reason at all. Radiant energy and radiant momentum deserve the 

 same standing; it is admitted that the energy / is the energy which is 

 brought by the radiation in unit time to unit area of the plate which 

 blocks the wave-train, and with it the radiation brings momentum I/c 

 in unit time to unit area of the plate. The density of radiant energy 

 in the wave-train is obviously /, c, the density of radiant momentum 

 is / f=. ' 



Now let that tentative idea, that radiant energ\' of tiie fretjuency v 

 is emitted and absorbed in packets of the amount hv, be completed 

 by the idea that these packets tra\el as entities from the place of 

 their birth to the place of their death. Let me now introduce the 

 word "quantum" to replace the alternative words packet, or unit, or 



