One of the most common atmospheric optics are without doubt rainbows. Formed due to reflection and refraction of sun light within the thousands of water droplets, the height of the bow is related with the height of sun on the opposite side. A low sun on the horizon gives a high bow and vice versa, so usually we see it in morning or evening. When reflection on water surfaces is omitted, two bows can theoretical show up: the bright primary bow and somewhat fainter secondary bow. Color both bows have always the same sequence: in the primary bow from the outer to inner side: red, orange, yellow, green, blue, indigo and violet while in the secondary bow these colors are reversed.

The picture shows piece of a the primary bow  which was taken on morning. The bow seems to end on a plane from Poland which waited  for clearance to take-off on runway 20 of Brussels Airport.      

More pictures of optics via link