D O C . 6 I N D I C E S O F R E F R A C T I O N F O R X - R A Y S 3 9
6. “Is It Possible to Determine Experimentally the
X-Ray Refractive Indices of Solids?”
[Einstein 1918i]
Received 21 March 1918
Published 30 June 1918
In: Deutsche Physikalische Gesellschaft. Verhandlungen 20 (1918): 86–87.
A few days ago I received from Herr Professor A. Köhler (Wiesbaden) a short pa-
per1
which describes a conspicuous phenomenon in X-ray pictures which has not
yet found an explanation. The reproduced photographs—mostly depicting human
extremities—exhibit along the contours a bright fringe of about 1 mm in width
where the plate seems to have received more radiation than in the (unshadowed) en-
vironment of the X-ray picture.
I would like to draw the attention of colleagues to this phenomenon and add
that this phenomenon is probably based on total reflection. We have to expect,
according to the classical theory of dispersion, that the index of refraction for
X-rays is close to 1, but in general still different from 1. is smaller or larger
than 1, depending upon the preponderant influence of electrons upon dispersion
with eigenfrequency smaller (or larger, resp.) than the frequency of X-rays. The
difficulty in the determination of is due to the fact that is very small
(about ). But it is easy to understand that with an almost grazing incidence of
X-rays—for the case —demonstrable total reflection must occur.
If we introduce into the refraction law
the complementary angles and , where and are
very small angles, and finally set
1A.
Köhler, Fortschritte auf dem Gebiet der Röntgenstrahlen 24 (1916), 236. (See Lucas
Gräfe & Sillem, Hamburg.)
[p. 86]
[1]
n
n
n n 1) – (
10–6
[2]
n 1
sinϕ-
sinϕ′
------------ n =
ψ 90 ϕ – = ψ′ 90 ϕ′ – = ψ ψ′