Say you have a rotating mirror and a fixed laser beam pointed towards the mirror such that the reflection of the beam changes direction.
Can you say that the light beam has been displaced?
Why or Why not?
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Displacement in this context means "the action of moving something from its place or position". When you reflect the beam you are changing the angle of the reflected beam, but you are not changing its origin at the reflection point. So calling this a displacement seems odd. Redirection, yes, but not displacement.
You are talking about lateral displacement of light, and lateral shift.
The perpendicular distance between the incident ray and the emergent ray is the lateral shift.
When a ray of light is incident obliquely on a parallel sided glass slab the emergent ray shifts laterally .
Where RT=lateral shift.
I understand you are confused because we sometimes use terms as lateral displacement for that.