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

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In the case of Dr Sweeney's fibre laser, the beam will have a wavelength of 1.5 microns, making it part of the infra-red spectrum.

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The second-easiest, however, is to delve further into the infra-red spectrum.

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They created a material that excluded light with a wavelength of one millimetre (the long end of the infra-red spectrum), using an array of aluminium-oxide posts that were spaced evenly across a grid.

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For example, iron-gall ink, commonly used on ancient documents, is transparent to infra-red light and most visible in the ultraviolet region of the spectrum.

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