Tempered glass substrates for high-speed compact disks and disk drives are made by healing surface cracks that form during manufacture. The disks are \(10 \mathrm{~cm}\) in diameter, \(3 \mathrm{~mm}\) thick, and are illuminated from below using a \(\mathrm{CO}_{2}\) laser that locally heats surface. The laser can produce a heat flux on the order of \(10 \mathrm{~kW} / \mathrm{m}^{2}\). If the surface of glass must be heated to \(750^{\circ} \mathrm{C}\), what temperature must we heat the bottom of the substrate to?