When a light of given wavelength is incident on metallic surface, the minimum potential  needed to stop the emitted photoelectrons is 6 V. This potential drops to 0.6 V if another  source with wave length four times that of first one and intensity half of the first one is  used. Wavelength of first source is _____ $\times {10}^{-7}\text{m}$.  (take $\frac{\text{hc}}{\text{e}}=1.24\times {10}^{-6}{\text{JmC}}^{-1}$  )