Burning mirror and flask, 18th century.
© Science Museum / Science & Society Picture Library
Heat is produced as sunlight is focused by a large convex lens, such as this example which is mounted on a bras pivot on a wooden stand. Heat can cause different substances to react together, or it can make a reaction happen more quickly. This can be demonstrated by focusing the heat on a flask containing ice, causing the ice to melt, which is a physical change. If the sunlight is focused onto paper, this can cause the paper to smoulder or even burst into flame, which is a chemical change. A physical change merely alters the physical state of a substance, whereas with a chemical change, new substances are formed.