When Is A Window A Prism?

Now that seems a very strange and difficult question to answer and you may be wondering what caused me to ask it.  Well, my reason for asking it was prompted when early one morning I saw this image projected on a wall in our hallway.  I knew immediately from where it emanated - 30 feet away from some new windows we had installed recently in our conservatory.

I snapped the shot which I thought was interesting.  Throughout the week we had noticed several such rainbows in many places, caused by the sun's rays being refracted just like a prism on striking the windows.  My mind recalled the famous acronym I learnt at school when white light entering a prism is split into all the colours of the rainbow.

Richard  Of  York  Gave  Blood  In  Vain

Red, Orange, Yellow, Green, Blue, Indigo, Violet

But windows panes are not prisms - they are just even sheets of glass, surely!  Not so these:

. . . . . . and another shot from the inside:

Ah!! yes . . . . . . you now know the answer to the question when is a window a prism?  When it has a bevel in it of course,  and there are now 9 bevels shapes in 3 fanlight windows we had installed.  The sunlight strikes the bevel and acts just like a prism, and because there are 9 bevelled diamonds the phenomena causes rainbow images to appear in interesting shapes.  The images move too as the Earth rotates, which is a bit eerie. I can't wait for the summer when we get more sun. But I hope the bevels do not act as magnifying glasses as well - remember using magnifying glasses when we were kids when we wanted to start fires in the garden, using the sun's rays on a sheet of paper?

The original three fanlight windows all had to be replaced because the seals had broken inside the double glazed units, causing a misty film which happens when such windows eventually break down.  I am an agent for a local window company so they fitted them for me two weeks ago.

Oh and just in the background of the last shot we can see my blue tit box - but no blue tit this time.  We see them frequently but they are flitting about all over the place and sometimes don't return for a few days.  Here is a close up shot of the bird house with the new bevel diamond showing refraction of the vegetation: