• that Mathis site is very interesting indeed. Can't say I understand half of it but is very interesting.

    You said you have a background in engineering. That should be more than adequate to understand his work -- it's all very clearly explained and he prefers to keep the math as simple and straightforward as possible. It's more transparent and less fudge-able that way. But here is a paper on what causes rainbows that requires no math: http://milesmathis.com/rainbow2.pdf