Решение.\[ \begin{align}
& E=e\cdot U,\ E=h\cdot \frac{c}{\lambda },\ e\cdot U=h\cdot \frac{c}{\lambda },\ U=\frac{h}{e}\cdot \frac{c}{\lambda },\ \Delta U={{U}_{1}}-{{U}_{2}}, \\
& \Delta U=\frac{h}{e}\cdot \frac{c}{{{\lambda }_{1}}}-\frac{h}{e}\cdot \frac{c}{{{\lambda }_{2}}},\ {{\lambda }_{2}}=2\cdot {{\lambda }_{1}},\ \Delta U=\frac{h\cdot c}{e}\cdot (\frac{1}{{{\lambda }_{1}}}-\frac{1}{2\cdot \lambda })=\frac{h\cdot c}{e}\cdot \frac{1}{2\cdot {{\lambda }_{1}}}. \\
& {{\lambda }_{1}}=\frac{h\cdot c}{e\cdot 2\cdot \Delta U}\ \ \ (1). \\
\end{align} \]
λ1 = 2,7∙10-11 м.