Answer: The wavelength of the incident light can be calculated using the formula λ = d * sin(θ), where d is the distance between the lines on the diffraction grating and θ is the angle between the diffracted beams. In this case, d = 1 mm and θ = 8° for the first order spectra.

Substituting the values in the formula, we get:

λ = (1 mm) * sin(8°) = 0.139 mm or 139 nm

Therefore, the wavelength of the monochromatic light incident on the diffraction grating is approximately 139 nm.