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KF5JRV > TECH 15.09.16 13:33l 69 Lines 3607 Bytes #999 (0) @ WW
BID : 2007_KF5JRV
Read: GUEST
Subj: Speed of Light Standard
Path: IW8PGT<CX2SA<N0KFQ<KF5JRV
Sent: 160915/1121Z 2007@KF5JRV.#NWAR.AR.USA.NA BPQ1.4.65
Speed of light standard
The krypton-86 discharge lamp operating at the triple point of nitrogen
(63.14 K, ) was the state-of-the-art light source for
interferometry in 1960, but it was soon to be superseded by a new
invention: the laser, of which the first working version was
constructed in the same year as the redefinition of the metre.
Laser light is usually highly monochromatic, and is also coherent
(all the light has the same phase, unlike the light from a discharge
lamp), both of which are advantageous for interferometry.
The shortcomings of the krypton standard were demonstrated by the
measurement of the wavelength of the light from a methane-stabilized
helium\endash neon laser ( 3.39 nm. The krypton line was found to be
asymmetrical, so different wavelengths could be found for the laser
light depending on which point on the krypton line was taken for
reference. The asymmetry also affected the precision to which the
wavelengths could be measured.
Developments in electronics also made it possible for the first
time to measure the frequency of light in or near the visible
region of the spectrum, instead of inferring the frequency from
the wavelength and the speed of light. Although visible and
infrared frequencies were still too high to be directly measured,
it was possible to construct a "chain" of laser frequencies that,
by suitable multiplication, differ from each other by only a
directly measurable frequency in the microwave region. The
frequency of the light from the methane-stabilized laser was
found to be 88.376\u8201?181\u8201?627(50) THz.
Independent measurements of frequency and wavelength are, in
effect, a measurement of the speed of light (c = f\f1\'eb), and the
results from the methane-stabilized laser gave the value for
the speed of light with an uncertainty almost 100 times lower
than previous measurements in the microwave region. Or, somewhat
inconveniently, the results gave two values for the speed of
light, depending on which point on the krypton line was chosen
to define the metre. This ambiguity was resolved in 1975, when
the 15th CGPM approved a conventional value of the speed of
light as exactly 299792 m/s.
Nevertheless, the infrared light from a methane-stabilized laser
was inconvenient for use in practical interferometry. It was
not until 1983 that the chain of frequency measurements reached
the 633 nm line of the helium\endash neon laser, stabilized using
molecular iodine. That same year, the 17th CGPM adopted
the current definition of the metre, in terms of the 1975
conventional value for the speed of light:
The metre is the length of the path travelled by light in
vacuum during a time interval of 1/299,792,458 of a second.
The concept of defining a unit of length in terms of a time
received some comment, although it was similar to Wilkins'
original proposal in 1668 to define the universal unit of length
in terms of the seconds pendulum. In both cases, the practical
issue is that time can be measured more accurately than length
(one part in 1013 for a second using a caesium clock as opposed
to four parts in 109 for the metre in 1983). The definition in
terms of the speed of light also means that the metre can be
realized using any light source of known frequency, rather than
defining a "preferred" source in advance. Given that there are
more than 22,000 lines in the visible spectrum of iodine, any
of which could be potentially used to stabilize a laser source,
the advantages of flexibility are obvious.
73, Scott kf5jrv
KF5JRV @ KF5JRV.#NWAR.AR.USA.NA
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