The observation of phenomena like diffraction (the failure of light rays to follow straight lines when passing very close to objects or through very small holes) had suggested to Thomas Young and Augustin Fresnel that light was some sort of wave and that it did not travel in straight lines when forced to squeeze through small holes because the holes were smaller than its wavelength. But no one in the early nineteenth century knew what light was a wave of; only with the work of James Clerk Maxwell in the 1860s did it become clear that light was a wave of varying electric and magnetic fields.
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