Evaluating the Quantum. 

To confirm the Einstein law, it is first necessary to calculate the number of photons in the beam of light which is to effect chemical change. Mathematically this number is: Nhv = Nhc/A = 2.859 X (10)5 /A kilocalories (Cal.) per mol. = I Einstein or 1 mol. of photons, where N = Avogadro's number, h = Planck's constant, v = frequency, c = speed of light in cm. per sec., and A = wave length. On this basis, a photon of wave length 4000 A (violet) has energy of 71 Cal., and one of 7500 A (red) has 38 Cal. Since many molecules must acquire perhaps 50-100 Cal. of energy before they will react rapidly (energy of activation), it can be seen why red light is often inefficient in causing chemical reactions. Thus, one sunburns in ultra-violet light of wave lengths below 3100 A (95 Cal.), a light which is absorbed by ordinary window glass. People living in glass houses will not sunburn. 

Light Sources. 

Obviously, light of a single wave length, that is, monochromatic light, is desired. For ultraviolet light, the available light sources, when properly filtered with special glasses to absorb other light waves, are: (a) the quartz-mercury vapor lamp, giving an intense resonance line at 2537 A, a"d other useful lines at 2830, 3130, 3660, and 4060 A; (b) spark spectra from zinc or aluminum (3120 A); and (c) for the visible light region, a tungsten-filament lamp giving a continuous spectrum from which small regions may be isolated by use of filters. The reaction vessel may be made of glass, which is transparent through the visible range down to about 3200 A; of quartz, which, when new, is transparent down to 1800 A, but becomes opaque below about 2200 A; or of cells of fluorite for the 1000 A to 2200 A range. 

Measuring Number of Photons. 

Direct measurement of the number of calories in a beam of light of known wave length will allow a calculation of the number of photons. The light beam may be caught on: (a) a thermopile, as of B1-Ag; (b) a photocell; or (c) a chemically-sensitive system. In the first two cases the energy received is compared with a light beam of standard intensity, either a carbon filament lamp from the United States Bureau of Standards, or a Hefner amyl acetate lamp which, when constructed according to fixed specifications, delivers a fixed light intensity. In case (c) a so-called chemometer, or actinometer, measures the number of light quanta in the beam. Draper (1846), Bunsen and Roscoe (1857), and Chapman (1906) developed such a device utilizing the combination of hydrogen with chlorine. Elder (I879) developed the photochemical reaction:

 LIGHT + 2HgCl2 + (NH4)2C204
                2NH4C1 + 2C02 + Hg2Cl2 

the weight of the Hg2Cl2 formed being indicative of the light intensity. A still more convenient actinometer employs the photodecomposition of oxalic acid (H2C2O4) in the presence of uranyl salts, the undecomposed, residual (H2C2O4) being titrated with KMnO4. 

Confirmation of the Einstein Law. 

If the law is valid, the value N/hv (number of molecules reacting per quantum) should be equal to unity. Chemists tested this law (1905-1915), and generally confirmed it. Some reactions, however, definitely appeared to deviate widely from the law. It soon became apparent that the cause of this deviation was secondary reaction, but that the initial step obeyed the Einstein Law.