A system of physical chemistry - Index of


A system of physical chemistry - Index of


when expressed in terms of the difference of the frequencies between

the "edges" of the strip. Let us suppose that the strip is chosen in

the middle of the visible spectrum. The mean value of X is then o-6/i.

or 6 X 10"^ cm., the limiting value to be ascribed to d\ being io~^ cm.

The frequency difference dv which corresponds to d\ is given by—

That is, for the case considered,


dv = " ri, • dX.

, ^ X 10^*' X 10"-*' ,.

36 X ^" 10

Hence, in the region of the spectrum referred to, the smallest width

to which we can attach a physical meaning corresponds to a frequency

difference of the order 10^**. The actual or absolute frequency in the

middle of this is

strip of the order 5 x lo^^

In bolometric or spectro-photometric determinations we measure

the energy AS radiated per second by a small region or strip of a continuous

spectrum which lies between X and X + AX. Since S = 3 x 10^**


AE . .

and therefore AS = ^ x lo^^ AE, we can calculate —r which in turn is

^ AX

identical with E^. We now wish to express the radiation formula of

Planck in such a way as to allow of convenient comparison between E'^

observed and its value as given by the formula. To do this let us rewrite

the formula already given :—


1 \

Remembering that u,4v = Ea

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