Chapter 5, Problem 42P

Until recently, it was thought that ${\text{Ca}}^{-}$ was unstable, and that the Ca atom therefore had a negative electron affinity. Some new experiments have now measured an electron affinity of $+\text{2}.0{\text{ kJ mol}}^{-\text{1}}$ for calcium. What is the longest wavelength of light that could remove an electron from ${\text{Ca}}^{-}$ ? In which region of the electromagnetic spectrum does this light fall?