Interactive Primary Newsletter 31 Under pressure

It‘s in a book, it must be true! The balloon and coat hanger is not the only questionable demonstration that we have found in publications. For example, there are a couple of real duffers on investigating light. One of which uses a glass bowl filled with water. The bowl is placed on a sunny window. A mirror is placed in the bowl at an angle of about 45 degrees, a sheet of A4 paper is set up opposite the mirror. “Eureka!” (to now misquote Archimedes) - a rainbow, it’s claimed, is formed on the paper. The second activity purports to be even easier. On the same sunny windowsill, place a glass of water with its base just over the edge of the sill. A sheet of A4 paper is placed on the floor so that the light from the glass is projected onto the paper. You’ve guessed it, the light falls on the paper as a rainbow. We have tried both experiments over a number of years with little success. However we recently tried again for this Newsletter and it did work. The reason being we tried around 9 o’clock on an Autumn morning when the sun was low in the sky. It would seem the best time to carry out this experiment would be when the sun is not too high in the sky perhaps on a bright sunny winter’s day. We’ve noticed also that you will get the big, semi-circular rainbows early in the morning (Fig.8) or late in the evening. As midday approaches the ones you observe are likely to be flatter as only the top of the ‘bow is visible. Our advice would be if you want to split light into colours use a prism. They are not too expensive and can be had from most suppliers of primary equipment. Fig.8 - Somewhere...over the....   If you disagree with any of this or perhaps have had success with any of the activities criticised here, we would like to hear from you. Equally, if you’re having trouble with any investigations from published works, or the web, we are here to help. Such technical support is one of SSERC’s major functions. Meantime, if you are forced to choose between the Tommy Cooper repertoire of science tricks and that old uncle’s - go for the uncle every time. Science teaches us, amongst other things, to be sceptical. It’s just as well.