Laboratory Glass-Working for Scientists - Sciencemadness Dot Org

©ENERAL PHYSICAL PROPERTIES OF GLASS
Transmission of Light
A 1-mm thick sheet of Phoenix glass will transmit 90 per cent or more
Of the light incident on it, for wavelengths of 350 millimicrons to
Almost 2 microns. In the infra-red region a strong absorption occurs
at 3 microns and little transmission beyond 4 microns. In the ultraviolet
region increasing absorption occurs as the wavelength falls
below 350 millimicrons and very little transmission occurs below 270
millimicrons. The transparent variety of vitreous silica (fused quartz)
has very superior optical properties, and is widely used in photochemical
and optical researches. In the ultra-violet region it transmits
at high efficiency down to 1850 Angstrom units (185 millimicrons).
'Quality O.H. VitreosiV of The Thermal Syndicate Ltd is
A special optical quality in which the absorption band at 2400
Angstrdrns has been eliminated. A special quality of fused quartz is
also available which transmits infra-red up to 3*5 microns approximately
('I.R. quality VitreosiV of The Thermal Syndicate Ltd). In this
Vitreosil the absorption band at 2*7 microns has been much reduced.
The Stress-Optical Coefficient
It IS not usual to take quantitative measurements of the strain in glass
apparatus made for research; when a strain-viewer is used (p. 43)
qualitative observations are normally made. Quantitative measure-
Kits can be made when the stress-optical coefficient is known. The
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involved requires a knowledge of the optical behaviour of
doubly refracting materials and depends on the fact that a ray of
p!ane»polarized light entering strained glass is broken into two rays—
the 'ordinary ray' and the 'extraordinary ray'—vibrating at right
•llgles to each other. For glass subject to simple axial tension or
Compression, the extraordinary ray vibrates in the plane which
includes the axis of the stress. The birefringence of strained glass is
proportional to the strain, and thus to the stress. The stress-optical
OOefficient is the maximum double refraction or birefringence obgerved
in polarized sodium light for 1 cm path length when there is a
Uniform stress of 1 kg/cm 2 . It is expressed either in wavelengths of
•Odium light or in millimicrons. This coefficient varies from one
glass to another; it is around 3*5 millimicrons, or 0*006 wavelengths
of sodium light A. JOHANNSEN (1918) has given an account of
methods for determining double refraction, and very valuable data
for practical work are given by J. H. PARTRIDGE (1949).
Resistance to Chemical Actions
General Chemical Properties of Glass
Vittoous silica is the most chemically inert glass for most purposes.
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