Caustics are curved surfaces of light that are created when a light source is reflected off a curved surface. They are often seen in the form of a rainbow or a halo around a light source. The term caustics comes from the Greek word 'kaustikos' which means 'to burn'. Lyes, which is a 4 letter solution to the crossword, is an alkaline solution made from ashes and water that can be used to clean and bleach fabrics.