3 · IGCSE Physics
Reflection in a plane mirror
Drag the object O and the eye to see how light reflects from a plane mirror, where the virtual image forms, and why the angle of incidence always equals the angle of reflection.
Drag O or the eye. Keyboard: focus the diagram and use the arrow keys.
i = r (both measured from the normal)
image distance = object distance
image: virtual, upright, same size
image distance = object distance
image: virtual, upright, same size
Object distance:
Image distance:
Angle of incidence i:
Angle of reflection r:
Try this
- Drag the eye from the top to the bottom of the picture. Watch i and r: do they ever differ?
- Set the object distance to 30.0 cm, then drag O towards the mirror. How does the image distance change?
- Move the eye far up or down until it can no longer see the image. Where does the reflected ray miss the mirror?
Common error: measuring angles from the mirror surface instead of from the normal – always draw the normal at 90° to the mirror first, and show rays behind the mirror as dashed lines because no light actually travels there.