1. Figure 1 shows a laser beam of wavelength 550 nm incidental normally on a diffraction grating.

The LDR of a scanning photometer is moved across the diffracted beam and produces the scan shown in Figure 2.This shows the central bright fringe with one further maximum (the first order image) on each side of it. The distance from the diffraction grating to the LDR is 150 mm.

Figure 1

Figure 2

What is the correct order of magnitude for the number of lines per mm on the diffraction grating?