Reflection of light (spec 3.14-3.16)
Angle of incidence = angle of reflection.
The law (spec 3.15). When a wave reflects off a flat surface, the angle of incidence and the angle of reflection are equal — both measured from the NORMAL (the line drawn perpendicular to the surface at the point of incidence).
Drawing a reflection ray diagram (spec 3.16).
- Draw the mirror as a straight line, with shading (hatching) on the back side.
- At the point where the ray hits, draw the NORMAL — a dashed line perpendicular to the mirror.
- Mark the angle of incidence between the incident ray and the normal.
- Draw the reflected ray on the other side of the normal at the SAME angle .
- Add arrows to show the direction of travel of both rays.
Common pitfall. Examiners deduct marks when candidates measure angles from the SURFACE rather than from the normal. Always draw the normal first.
Image in a plane mirror. The image is virtual (behind the mirror), upright, the same size as the object, and laterally inverted (left-right flipped). The image distance behind the mirror = the object distance in front.
- Both angles measured from the NORMAL.
- Angle of incidence = angle of reflection.
- Image in a plane mirror is virtual, upright, laterally inverted.