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25)
Use dsinθ = nλ
So given, θ = 45 and n = 3
so d x sin45 = 3λ ---> equation 1
We need to find highest order yes so check at which values of θ the sinθ has its maximum value, that is sin90 = 1
so we get other equation :
d x sin90 = nλ ---> equation 2
Now divide Equation 1 by 2 so that d and λ gets cut and you get n
so sin45 = 0.7 and sin90 = 1 ; (d x 0.7 = 3λ) / (d = nλ) this will give :
0.7 = 3/n
so n = 3 / 0.7 ≈ 4
so you get your answer as 4th