Explanation:
The number of images formed by a pair mirrors kept at an angle [tex]\alpha[/tex] is
[tex]n=\frac{360}{\alpha } -1[/tex] if [tex]\frac{360}{\alpha }[/tex] comes even.
[tex]n=\frac{360}{\alpha }[/tex] if [tex]\frac{360}{\alpha }[/tex] comes odd.
where [tex]n[/tex] is the number of images formed.
In our question,
[tex]\alpha =120^{0}[/tex]
[tex]\frac{360}{\alpha } =\frac{360}{120}=3[/tex]
It is odd,so [tex]n=\frac{360}{120}=3[/tex]
