Optics
Plane mirrors
Verify the law of reflection for each incident ray and confirm that a plane mirror forms a virtual image symmetric to the object about the mirror plane, same size and reversed in depth.
Equipment
- Plane mirror with a point object and the normal drawn
- “Incident angle” slider for the incoming ray
- Rotatable 3D figures showing incident, reflected and virtual rays
- Second diagram emphasizing the symmetric image position
Procedure
Measure the incident and reflected angles
Drag the “Incident angle” slider from 5° to 80°: at each setting compare the reflected ray's angle to the normal and check . Recall that both angles are measured from the normal at the point of incidence, not from the mirror surface.
Locate the virtual image via ray extensions
Move to the second figure: watch the backward extensions of the reflected rays meet at one point behind the mirror — the virtual image. Check on the figure that image and object are equidistant from the mirror, so the object–image separation is twice the object–mirror distance.
Distinguish real and virtual images
Rotate the figure to see the reflected beam entering an observer's eye: the rays diverge as though emitted from the image behind the mirror. State the test for a virtual image — it cannot be projected on a screen — and deduce the “depth reversal” that makes text on a shirt read backwards in the mirror.