110
EINSTEIN ON PHOTOCHEMICAL EQUIVALENCE
color and
not
on
the
intensity
of
the
light? Why
are
rays
of shorter
wavelengths
generally
more
effective
chemically
than
those of
longer wavelengths?"[4]
In 1905
Einstein had shown that
the
light quantum hypothesis
also
provides
a
qual-
itative
explanation
of
the
ionization of
gases
by
ultraviolet
light.[5]
Taking
this
approach
as a
starting point,
Johannes Stark
published a paper
in 1908
which
treats
photochemistry
on
the
basis of
the
light quantum hypothesis,
and which contains
statements
resembling
Einstein's
law
of
photochemical equivalence
of
1912.[6]
When
Stark later claimed
priority
for
the
discovery
of
the
law,
however,
Einstein character-
ized
it
as
"a
quite
obvious
consequence
of the
quantum hypothesis."[7]
While
the
light quantum hypothesis
was
strikingly
successful
in
some areas
of
experimental
research,
photochemistry
turned
out to be difficult terrain for
its
appli-
cation. The
light quantum hypothesis
could indeed
account
for
the
dependence
of
photochemical
reactions
on
the
color of
the
light.
It
offered
no
explanation, however,
for
the
existence of
intensity
thresholds
in
photographic processes,
where
the
intensity
of
the
incoming
radiation seemed
to
come
into
play.
Einstein concerned himself
with
this
problem
in
1909, shortly
after
he
delivered
his
talk
to
the
81st
meeting
of
the
Gesellschaft Deutscher Naturforscher
und
Arzte
in
Salzburg,
in which he
formulated
the
questions
cited above.
He
apparently
asked
the
experimental physicist Edgar
Meyer
to
find out
more
about
the state
of research
on
the
sensitivity
threshold
in
photography
and
concluded from
his
report
that
photography cannot be
used in decid-
ing
the
question
whether
or
not
small intensities of
light
are photochemically
active.[8]
To his
friend Michele
Besso,
who had drawn
his
attention
to
a
recent
publication on
this
topic,
Einstein
wrote:
"Photochemical
processes
thus
appear
to be
unsuitable for
testing
the
quantum hypothesis."[9] Although
for
a
time
Einstein
apparently
did
not
pursue
the
question
any
further, he
remained unconvinced
by
experimental
evidence
indicating
that
light
of
low
intensity is photochemically
ineffective.
He
tentatively
explained
the
existence of
an
apparent sensitivity
threshold
by a
dynamic equilibrium
between
the
photochemical process
induced
by
exposure
and the
inverse of
this
pro-
cess.[10]
This
idea
of
a
dynamic equilibrium
may
have been crucial for
the
use
of the
law
of
mass
action
in
the
argument
later
published
in Einstein
1912b
(Doc. 2).
[4]"Warum
hängt
es
nur von
der
Farbe,
nicht aber
von
der Intensität
des
Lichtes
ab,
ob
eine
bestimmte
photochemische
Reaktion eintritt oder nicht? Warum
sind
die
kurzwelligen
Strahlen
im
allgemeinen
chemisch wirksamer
als die
langwelligen?"
(Einstein
1909c
[Vol. 2,
Doc. 60],
p.
490).
[5]See Einstein
1905i (Vol.
2,
Doc.
14), §9.
[6]See
Stark
1908a,
in
particular
p.
893, and
Stark
1908b.
For
a
historical
account,
see
Hermann
1971,
pp.
79-80.
[7]"Eine
ganz
selbstverständliche
Folgerung
der
Quantenhypothese" (see
Einstein
1912g
[Doc. 6]).
[8]See
Einstein
to Edgar Meyer,
29
October
1909
(Vol. 5,
Doc.
182),
in
which Einstein
thanks
Meyer
for
his
assistance.
[9]"Photochemische Prozesse scheinen deshalb
ungeeignet
zur
Prüfung
der
Quan-
tenhypothese"
(Einstein
to
Michele
Besso,
17
November
1909
[Vol. 5,
Doc.
187]).
[10]See Einstein
to
Edgar Meyer,
29
October
1909
(Vol. 5,
Doc.
182).
Previous Page Next Page