JAN HINSCH / SCIENCE PHOTO LIBRARY. JAN HINSCH / SCIENCE PHOTO LIBRARY.
This particular type of fibre is known as a monomode. The feint impression of the innermost core of the fibre is visible as a darker shade of red (its refractive index is different from the surrounding material): it is along this inner core that the light signals travel. The additional external tube is the fibre's protective cladding which takes no part in optical transmission but protects the delicate fibre from mechanical damage. Differential interference contrast illumination, magnification: X 64 at 35mm size. Reference: MICROCOSMOS figure 8.27, page 165.
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