This photo was taken on a train travelling south to Hastings. The flat windows had horizontal and vertical scratches, perhaps from cleaning. The arc that is illuminated by the sun appears to be centred on the vanishing point of the axis of the carriage. The Sun lies on the arc. Why this arc?

The optical effect can be seen by carrying out the following procedure:

Place two panels containing a regular pattern of hexagonal holes of about 4mm diameter one in front of the other, separated by at least 1 metre. Approach the panels from some distance away. As you get closer you will see a magnified image of the holes. The magnification varies with distance from the panels.

This phenomenon can be observed when approaching bridges over the M25 motorway, as shown in the attached photograph.