Determine the length of the light wave incident on the diffraction grating, each millimeter of which has 400 lines.

Determine the length of the light wave incident on the diffraction grating, each millimeter of which has 400 lines. The diffraction grating is located at a distance of 25 cm from the screen. When measuring on the screen, it turned out that the distance between the third and zero lines is 12 cm?

The solution of the problem :
d * sin φ = kλ
sin φ = tan φ = b / a, b = S / 2 = 0.137 m
λ = d * sin φ / k = d * b / k * a
λ = 2.5 * 10 ^ -6 * 0.137 / (3 * 0.25) = 4.6 * 10 ^ -7 (m)
Answer: λ = 4.6 * 10 ^ -7 (m)