Bước sóng:
$\lambda =\dfrac{a.i}{D}=\dfrac{2.0,2}{1}=0.4\,\,\left( \mu m \right)$
$\,\,\,\,\,\,\,k\lambda =k'\lambda '$
$\to \lambda '=\dfrac{k\lambda }{k'}=\dfrac{3.0,4}{k'}=\dfrac{1,2}{k'}$
Vì $\lambda '\,>\,\lambda $
Nên $k'\,<k$
$\to k'<3$
$\to k'\in \left\{ 1;2 \right\}$
Với $k'=1$
$\to \lambda '=\dfrac{1,2}{1}=1,2\,\,\left( \mu m \right)$
Với $k'=2$
$\to \lambda '=\dfrac{1,2}{2}=0,6\,\,\left( \mu m \right)$