Images show these full circular bows as static entities.   Yet rainbows and optics are alive, they move when you move, they sparkle, they shimmer, shift and shade with elusive ways.   So do these indoor studies - fixed images do less than justice to what the works express.

‘Rood Screen’, of many tilted planes, shows the invariance of rainbows; at least when the sun, or in this case the powerful generating light, is far enough away that its rays are near parallel.    The bow stays circular regardless of the tilt of the glass sphere sprinkled boards.   Move further away and the bow expands across the boards.   Or, more accurately, the boards shrink with distance across the invariant bow.   Move closer and a single board holds the bow – yet the bow keeps the same diameter.

If some vandal had dismantled the rood screen and spread its boards at all distances still the bow would remain invariant. Outdoor bows are equally invariant, the distance of the rain matters little.

Change the rain from water’s spheres to ones of glass and then the rainbow does change, in size and in the separation – not order – of its colours. Glass refracts more strongly than water and its bow is only some 21 degrees in radius, half that of an ordinary rainbow.

What if an extravagant artist sprinkled boards with spheres fashioned from diamonds? Sadly, that most refractive of substances would give no first order rainbow at all. To form a rainbow the parallel rays streaming into the sphere must at some position be deflected through a minimum angle of deviation. Rays cluster around that angle to form the bright rainbow's rim. Diamond's rays have no such minimum and therefore create no rainbow.