(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.