= Solution
A \b[high-pressure <Mercury> arc lamp] is a conventional source for <proximity photolithography>. An electric discharge excites <Mercury> vapour; radiative transitions produce strong spectral lines, selected by filters and delivered through collimating optics. Common lines are \b[$365\,\mathrm{nm}$ (i-line)] and \b[$436\,\mathrm{nm}$ (g-line)]. The finite mask-wafer gap gives near-field <Fresnel diffraction>, rather than the far-field approximation. Diffracted light spreads into nominally dark regions, limiting the minimum printable feature.
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