Solve this:
​Q21. Energy of an electron is given by E = – 2.178 × 10^–18 J $\left(\frac{{\mathrm{Z}}^2}{{\mathrm{n}}^2}\right)$.
        Wavelength of light required to excite an electron in an hydrogen atom from level n = 1 to n = 2 will be
             (h = 6.62 × 10^–34 Js and c= 3.0 × 10⁸ ms^–1)

      (a) 1.214 × 10^–7 m

      (b) 2.816 × 10^–7 m

      (c) 6. 500 × 10^–7 m

      (d) 8.500 × 10^–7 m