Reflective surfaces are, I suppose, the echo chambers of light and a reflection can be both faithful to the original as well as changing, amplifying, softening or distorting it. 
In this picture, the reflective surface increases both the amount of light and the amount of colour in the otherwise lightless grey pavement as well as giving an extra diluted dimension to buildings and people. It’s the same technique that Caravaggio uses to bring depth to this view of Venice and to integrate the flat surface of the water into the picture.
Van Eyk’s mirror (previous post) serves a different purpose – or different purposes. Firstly, it gives you a view from behind the main subjects, so that you see them from behind as well as from the front; secondly, you see the room looking out back wall as well in from the “fourth” wall. More significantly, it introduces a second scene – the scene behind the painter.
In a spirit of experimentation, I tried to mimic this technique using the tools at my disposal, whilst trying not to get trodden under foot (rumours that nobody goes to the National Gallery any more are greatly exaggerated). So, with apologies for the mirror which is a) not convex and b) pink …
