Cube3D 立方体数理Physics · Primary school / Middle school
How do you measure the angle of reflection?
A reflected ray does not choose an arbitrary direction. For an ideal plane mirror, the angle of reflection equals the angle of incidence. The essential detail is the reference line: both angles are measured from the normal, an imaginary line perpendicular to the mirror at the point where the ray meets it.
Start with the normal, not the surface
Picture a horizontal mirror with a vertical normal. If the source lies at a bearing of 135° counterclockwise from the right, it is 45° away from the normal at 90°. The incident ray travels from that source toward the mirror, and the reflected ray leaves at a bearing of 45°. Both measured angles are 45°.
A ray making 20° with the mirror instead makes 70° with its perpendicular normal. Its incidence and reflection angles are therefore 70°, not 20°. In the lab, the gold and green arcs mark angles relative to the dashed normal so that the choice of reference is visible.
Keep the source fixed and turn the mirror
Reset the lab to source bearing 135° and mirror angle 0°. Before changing anything, predict the outgoing direction if only the mirror turns counterclockwise by 10°. Move the mirror slider to +10° while leaving the source slider at 135°.
The normal now has bearing 100°, so the incidence angle is 135° − 100° = 35°. The reflected ray is 35° on the other side of the normal, giving outgoing bearing 100° − 35° = 65°. Its direction has changed by 20° although the mirror turned only 10°. This comparison requires the incoming direction to stay fixed.
Check your understanding
Choose normal incidence, with source bearing 90° and a horizontal mirror. What are the two measured angles, and why can the gold and green paths overlap?
Show the explanation
Both angles are 0°. Reflection sends the ray back along its incoming path in the opposite direction; overlapping paths do not mean that reflection has disappeared.
What the model assumes
The lab uses geometric optics with a flat mirror, two reflective faces and rays confined to one plane. It does not calculate brightness, absorption, diffraction or surface roughness. At exactly grazing incidence, the ray lies along the mirror and is shown as a limiting case, not an ordinary intersection. The experiment requires an active Family Plus or Teacher pass; this explanation is public.