For the 4th harmonic, L=2λL = 2\lambdaL=2λ
4m=2λ ⟹ λ=4m24m = 2\lambda \implies \lambda = \frac{4m}{2}4m=2λ⟹λ=24m
=2m2m2m
Recall, v=fλv = f\lambdav=fλ
1=f×2 ⟹ f=121 = f \times 2 \implies f = \frac{1}{2}1=f×2⟹f=21
= 0.50Hz