How many times have we read that on Birdforum?
However, none of us consumers has much idea what glass is used in our optics and what is "better glass" anyway?
Some manufacturers of astronomical telescopes advertise low dispersion glass such as FPL-53 used in their objectives, Kowa use Calcium fluoride (an artficially grown crystal, not glass) in their 55, 88 and 99 scopes and Zeiss used fluoride doped glass in the FL binoculars, but that's about the extent of information available to us.
Glass is basically Silicon dioxide (quartz) with the addition of other oxides, halogenides etc. to give it different refractive and dispersive properties. It can be considered as a supercooled liquid as it has no defined melting point, but the viscosity at ambient temperatures is so enormously high that there would be no measurable change in dimensions over decades.
Apart from transparency, the two most important properties of optical glasses are their refractive indices and Abbe numbers.
When light enters a dense medium (water, glass) from a less dense medium (vacuum, air) it is slowed and the refractive index of a glass is the ratio of the speed of light in vacuum to the speed of light in the glass. As the speed of light in vacuum is about 300.000 km/s, this would be rather difficult to measure!
One could imagine a wave front hitting a glass surface at an angle (angle of incidence is that to the perpendicular) and as it is slowed it is deflected towards the perpendicular (angle of refraction). The refractive index is the ratio of the sines of the angles of incidence and refraction.
However, refraction also depends on the frequency of the light and short wavelengths (blue) are refracted more than longer wavelenths (red), so if the optical designer has been less successful in correcting this, we see colour fringing (chromatic aberration) in our binoculars.
The Abbe number expresses the differences in the refactive indices of an optical glass at both ends of the spectrum with respect to the refractive index in the middle of the visible spectrum. The higher the number, the lower the dispersion.
Crown glasses generally have a relatively low refractive index and low dispersion (high Abbe no.) and flint glasses a high refractive index and high dispersion (low Abbe no.).
A crown glass example is Schott BK-7 (Ref. 517642), which indicates a refractive index of 1,517 and an Abbe no. of 64,2.
By combining optical glasses it is possible to attain refraction while minimizing dispersion and designers have a wide choice available with more than 120 varieties available from Schott alone, some in high transmission versions and others with extremely tight tolerances in refractive index and a minimum amount of bubbles in the glass blanks.
By combining a positive crown element and a negative flint element achromatic correction (2 colour wavelengths) is possible but apochromatic (3 wavelengths) or super-apochromatic correction (4 wavelengths) usually demands complex designs with exotic (and expensive) glass formulations.
For those interested, here is a useful database Refractive index of CH4 (Methane) - Rollefson, which includes not only Schott, but other optical glass manufacturers.
Corrections or additions to the above welcome, as always.
John
However, none of us consumers has much idea what glass is used in our optics and what is "better glass" anyway?
Some manufacturers of astronomical telescopes advertise low dispersion glass such as FPL-53 used in their objectives, Kowa use Calcium fluoride (an artficially grown crystal, not glass) in their 55, 88 and 99 scopes and Zeiss used fluoride doped glass in the FL binoculars, but that's about the extent of information available to us.
Glass is basically Silicon dioxide (quartz) with the addition of other oxides, halogenides etc. to give it different refractive and dispersive properties. It can be considered as a supercooled liquid as it has no defined melting point, but the viscosity at ambient temperatures is so enormously high that there would be no measurable change in dimensions over decades.
Apart from transparency, the two most important properties of optical glasses are their refractive indices and Abbe numbers.
When light enters a dense medium (water, glass) from a less dense medium (vacuum, air) it is slowed and the refractive index of a glass is the ratio of the speed of light in vacuum to the speed of light in the glass. As the speed of light in vacuum is about 300.000 km/s, this would be rather difficult to measure!
One could imagine a wave front hitting a glass surface at an angle (angle of incidence is that to the perpendicular) and as it is slowed it is deflected towards the perpendicular (angle of refraction). The refractive index is the ratio of the sines of the angles of incidence and refraction.
However, refraction also depends on the frequency of the light and short wavelengths (blue) are refracted more than longer wavelenths (red), so if the optical designer has been less successful in correcting this, we see colour fringing (chromatic aberration) in our binoculars.
The Abbe number expresses the differences in the refactive indices of an optical glass at both ends of the spectrum with respect to the refractive index in the middle of the visible spectrum. The higher the number, the lower the dispersion.
Crown glasses generally have a relatively low refractive index and low dispersion (high Abbe no.) and flint glasses a high refractive index and high dispersion (low Abbe no.).
A crown glass example is Schott BK-7 (Ref. 517642), which indicates a refractive index of 1,517 and an Abbe no. of 64,2.
By combining optical glasses it is possible to attain refraction while minimizing dispersion and designers have a wide choice available with more than 120 varieties available from Schott alone, some in high transmission versions and others with extremely tight tolerances in refractive index and a minimum amount of bubbles in the glass blanks.
By combining a positive crown element and a negative flint element achromatic correction (2 colour wavelengths) is possible but apochromatic (3 wavelengths) or super-apochromatic correction (4 wavelengths) usually demands complex designs with exotic (and expensive) glass formulations.
For those interested, here is a useful database Refractive index of CH4 (Methane) - Rollefson, which includes not only Schott, but other optical glass manufacturers.
Corrections or additions to the above welcome, as always.
John