What is the highest spectral order illuminated with 633-nm laser light?

Given values:
Number of slits per centimeter, $N=6500\text{ slits/cm}$
Wavelength, $\lambda =633\text{ nm}$
Angle of diffraction, $\theta ={90}^{\circ }$
The phenomenon of bending of the light waves around the cornerns of obstacle is called diffraction.
The condition for diffraction maximum is given by,
$n\lambda =d\sin \theta$
Where,
n= Order of diffraction
$\lambda =$ Wavelength of light
d= Separation between the slits
$\theta =$ Angle of diffraction
The number of slits per centimeter is equal to the separation between the slits.
Number of slits per cm, $N=\frac{1}{d}$
Substituting equation
$$\begin{gathered} n\lambda =\frac{\sin \theta }{N} \\ n=\frac{\sin \theta }{\lambda N} \end{gathered}$$
Substituting number of slits per centimeter, wavelength and angle of diffraction in equation
$$\begin{gathered} n=\frac{\sin {90}^{\circ }}{6500\times {10}^2\text{ slits/m}\times 633\times {10}^{-9}\text{ m}} \\ =2.43 \\ \approx 2 \end{gathered}$$