**Given:**

$θ = 30^o$

$n = 2$

$λ = 5 × 10^{-5} cm$

**To find:**

No. of lines/cm = $\frac{1}{d}$

**Solution:**

We know that for principal maxima

$d sinθ = nλ$

$\frac{1}{d} = \frac{sin θ}{nλ}$

$\frac{1}{d} = \frac{sin 30}{2 × 5 × 10^{-5}}$

$\frac{1}{d} = 5000$

No. of lines per cm is 5000