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The theoretical discovery of transparent particles that break the previously accepted limit of visibility opens a new door in the search for perfect transparency.

Transparent particles with extraordinarily high refractive indices can become almost invisible at wavelengths longer than the particle size, a Singapore's Agency for Science, Technology and Research (A*STAR)-led theoretical study has shown. The discovery challenges the accepted wisdom around the limits of light scattering and visibility, and could lead to a new class of 'invisible' materials.

The scattering of sunlight from gas molecules in the atmosphere is what makes the sky look blue, allowing us to effectively see what would other be a transparent medium. This process, known as Rayleigh scattering, occurs when molecules or particles are smaller than the wavelength of light that hits them. It has long been accepted that all particles undergo Rayleigh scattering, and that the minimum amount of scattering occurs when the refractive index -- a measure of the 'slowness' of light passing through a medium compared with a vacuum -- is less than two. Water, air and glass all meet this condition, suggesting that the Rayleigh scattering that makes the sky blue is the least visible state physically achievable.

Boris Luk'yanchuk and colleagues from the A*STAR Data Storage Institute, in collaboration with researchers from the Australian National University, have now upset this status quo with the discovery that Rayleigh scattering can be suppressed in transparent particles at wavelengths longer than the particle scale if their refractive index is extraordinarily high.

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Category: Science