Shape of Reflection depends on shape of mirror

The shape of the reflection of a light source from a plane mirror depends on the shape of the mirror. Thus if a square mirror is illuminated by a candle, an arc lamp, a halogen lamp (etc) then the reflection it casts will be square. In the case of an arc lamp the aperture the light is emitted from is circular making the effect more convincing to students. The most effective way to show this effect is with the sun, which is seen from Earth as being a circle. The reflection of this circle in a square mirror is seen as a square reflection. The reflection from a round mirror is seen as being round.