Two mirrors meet at right angles. A ray of light is incident on one end at an angle of ${30}^o$ as shown in figure. Draw the reflected ray from the second mirror.

At Mirror OA,

Angle of incidence = angle of reflection = ${30}^o$

$\angle CAB={30}^o$

$\angle CAB+\angle CBA+\angle ACB={180}^o$

${30}^o+{90}^o+\angle ABC={180}^o$

$\angle ABC={60}^o$

At mirror OB,

Angle of incidence = angle of reflection = ${60}^o$

Note that,

$AC||OB$

Angle between incident ray and AC = Angle between reflected ray and OB $={30}^o$

Hence, first incident ray are last reflected ray in this case are parallel.