Answer:
402 nm .
Explanation:
Let the wave length of unknown light be λ .
For fifth order maxima the condition is
path difference at the point of fringe = 5 λ
For red light of 670 nm , for 3 rd order maxima
path difference at the point of fringe = 3 x wave length
= 3 x 670 nm
From these two equations
5 λ = 3 x 670 nm
λ = 3 x 670 / 5 nm
= 402 nm .
