Two plane mirrors are held parallel to each other and at an angle ${45}^{\circ }$ to the line joining them as shown in figure. What will be the angle between incident ray and emergent ray ( ray after reflection from a second mirror) ?

The correct option is B ${180}^{\circ }$
Angle of the incident light ray on plane mirror $M1$ is ${45}^{\circ }$. Reflected ray $AB$ made an angle ${45}^{\circ }$ with the normal $AM$. Angle of incident on Mirror $M2$ is ${45}^{\circ }$.
From Law of reflection
Angle of incidence $=$ Angle of reflection
Thus light from the second mirror gets reflected parallel to the incident ray at mirror $M1$.
Angle between incident and reflected ray is ${180}^{\circ }$.
Periscope is an instrument based on this principle. We can see objects behind the obstacles using this. Submarines have periscopes so that people inside can see what is approaching over the surface of water.