A gel is an apparently solid, jellylike material formed from a colloidal solution. By weight, gels are mostly liquid, yet they behave like solids. An example is gelatin.
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In fiber optics communications, a gel resembling petroleum jelly in viscosity is used to surround a fiber, or multiple fibers, enclosed in a loose buffer tube. This gel serves to lubricate and support the fibers in the buffer tube. It also prevents water intrusion if the buffer tube is breached. Gels are also used in fiber-optics as index-matching materials.
Source: from Federal Standard 1037C and from the FAA Glossary of Optical Communications Terms
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In Filter (optics) and theatrical lighting a color gel is a transparent or translucent colored panel used to change the color of transmitted light.
Gel
I wouldn't agree that the type of gel used in fibre optic cables resembles petroleum jelly in viscosity. The gels used for this application are typically similar in consistency to soft "hair gel", and are comprised of synthetic hydrocarbon oil gelled with either colloidal fumed silica, or various synthetic rubbers. The viscosity is highly dependant upon shear rate, and these materials do not "melt" as such. Petroleum jelly is solid at room temperature and melts at temperatures above about 70°C to form a Newtonian fluid.