But crystal fibres push the range of single-mode wavelengths right into the visible part of the spectrum, opening up huge new tracts of wavelength for telecommunications.

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This part of the infra-red spectrum, with wavelengths close to that of visible light, carries heat energy.

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Edison bulbs function by making a Tungsten filament very hot, and a hot object glows: radiating energy on a broad spectrum of wavelengths, only a small part of which is visible to the eye.

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