• question_answer 18) Two wavelengths of light ${{\lambda }_{1}}\,\,and\,\,{{\lambda }_{2}}$ are sent through Young's double slit apparatus simultaneously. What must be true about ${{\lambda }_{1}}\,\,and\,\,{{\lambda }_{2}}$ if the third order bright fringe of ${{\lambda }_{1}}$ coincides with fifth order dark fringe of${{\lambda }_{2}}$? A) $3{{\lambda }_{1}}=2{{\lambda }_{2}}$                    B) $2{{\lambda }_{1}}=3{{\lambda }_{2}}$C) $3{{\lambda }_{1}}=5{{\lambda }_{2}}$                    D) $5{{\lambda }_{1}}=3{{\lambda }_{2}}$

By theory ${{n}_{1}}{{\lambda }_{1}}=\left( 2{{n}_{2}}-1 \right){{\lambda }_{2}}/2$ $3{{\lambda }_{1}}=\left( 2\times 5-1 \right)\lambda {{a}_{2}}/2$ $3{{\lambda }_{1}}=\frac{9{{\lambda }_{2}}}{2}$