(a) For monochromatic light the resultant intensity is IR‌‌=I1+I2+2√I1I2‌cos‌θ ‌‌=2I+2l‌cos‌θ (∵I1=I2=I)...(i) For maximum intensity, θ=0∘ ‌IR=2l+2l‌cos‌0∘=4I‌‌(∵cos‌0∘=1) ‌ or ‌‌K=4I ‌ or ‌‌I=‌
K
4
For path difference λ∕3, phase difference, φ=2π×‌