The grille has 120 lines per 1 mm. It is known that we are seeing the 1st maximum. The angle at which we observe both

The grille has 120 lines per 1 mm. It is known that we are seeing the 1st maximum. The angle at which we observe both 1 maximums is 8 degrees. Find the wavelength.

Let’s write the equation:
Bsinφ = kλ
Where
B-lattice period
N – number of strokes

Let’s write B and substitute it in the original:
B = Δl / N
Δlsinφ / N = kλ

Find λ
λ = Δlsinφ / kN
λ = 10 ^ -3 * sin8 ° / 120 * 1 = 5.8 * 10 ^ -7 (m)

Answer: λ = 5.8 * 10 ^ -7 m