Solution :
For first minima of red
sinθ=λrdsinθ=λrd
For first maximum of other wave length
sinθ1=3λ′2dsinθ1=3λ′2d
But sinθ=sinθ1sinθ=sinθ1
=> 3λ12d=λd3λ12d=λd
λ1=23λ1=23 λrλr
=23=23 ×660×660
=440nm
~peace
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