Can light retrace its path?

Choose a glass shape, then reverse the ray. With the same media and corresponding angles, it follows exactly the same path back.

Light passing through glass A ray enters and leaves the glass. Direction arrows reverse along the same path when the direction is reversed.

Orange dashed line: forward path · Blue arrows: current direction · Grey dashed lines: normals · Purple values: angles measured from the normal.

Angles are measured from the normal. Reversing swaps the incident and refracted angles at each boundary: n₁ sin θ₁ = n₂ sin θ₂. Shapes are shown in cross-section; partial reflections are omitted.