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Within those cables there are tiny strands of fiber optic material, the width of an eyelash.

A silent protest had been planned too, which would have seen locals turn their back on the American visitors - a potentially publicly awkward situation and poor optics for Mrs Vance.

Immigration officials “need that optic, which is extremely cruel for the people being detained and deported,” Salas said.

But this can also lead to swelling in the brain and changes in the eye, including to the optic nerve, retina and even the shape of the eye.

When pulses of light travel along a fibre optic strand, tiny reflections sometimes bounce back along that line.