(a) The emission of light from a substance when its excited electrons move to a lower energy level. OR Distinct/separate bright lines of light of definite frequency/wavelength (emitted from vapourised atoms) (ii) Emission spectra is the pattern of light displayed when electrons jump from a higher energy level to a lower energy level, while absorption spectra is the displayed pattern of wavelengths absorbed by the gas. (b) \(\Delta\)E = E\(_i\) - E\(_f\) = -1.51 + 13.6 = 12.09 eV = 12.09 x 1.6 x 10\(^{-19}\) J = 1.934 x 10\(^{-18}\) J (ii) \(\Delta\)E- hf f = \(\frac{\Delta E}{h}\) = \(\frac{1.934 \times 10^{-18}}{6.6 \times 10^{-34}}\) f = 2.93 x 10\(^{15}\)Hz (ii) c = f\(\lambda\) \(\lambda\) = \(\frac{3 \times 10^8}{2.93 x 10^{13}}\) = 1.023 x 10\(^{-7}\)m (c) (I) Soft x-rays are those with longer wavelengths/lowNer penetrating/less energy while hard x-rays are those with shorter wavelengths/higher penetrating power more energy. (iii) To increase the electrical conductivity/reduce electrical resistivity of the semiconductor.