If light of wavelength of maximum intensity emitted from a surface at temperature $T_1$ is used to cause photoelectric emission from a metallic surface, the maximum kinetic energy of the emitted electron is $6\,\text{eV}$, which is $3$ times the work function of the metallic surface. If light of wavelength of maximum intensity emitted from a surface at temperature $T_2$ ($T_2 = 2T_1$) is used, the maximum kinetic energy of the photoelectrons emitted is: