I spent time in the Scottish hills last winter and on a couple of occasions I had cause to clean my glasses in a stream that originated from melting snow, effectively at 0 掳C. The water cooled the glass and its metal frame to such an extent that both lenses fell out. But how could this happen when, if I remember my school physics correctly, metal should contract more than optical glass because of a higher coefficient of expansion? Obviously this has never happened when I鈥檝e been walking around under normal conditions.
We received some very entertaining answers to this question, but we haven鈥檛 really nailed it yet. Several people called for more experimentation or wanted to know the coefficients of expansion for optical plastics so that they could be compared with those for metals 鈥� Ed
鈥� Your correspondent does not say whether he had put the glasses back on when the lenses fell out. If he had, the warmth of his body probably heated the frames more rapidly than the glass.
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If it happened while he was washing the glasses in the stream, then the rapidly falling temperature might have shrunk the metal frames and squeezed the lenses out.
鈥淭he rapidly falling temperature might have shrunk the frames and squeezed the lenses out鈥�
Doug Grigg, Cannonvale, Queensland, Australia
鈥� Unless the questioner鈥檚 spectacles were very old, his lenses would be made of plastic not glass. This has a coefficient of expansion many times that of steel.
Alan Hickman. Sleaford, Lincolnshire, UK
鈥� Glass lenses could not fall out of a glasses frame as a result of thermal contraction, but most lenses today are polymers. Steel has a linear thermal expansion coefficient of about 2 脳 10-5 per 掳C, while that for polycarbonate is about 7 脳 10-5 per 掳C. On cooling, the lens will contract inside the frame, but the total difference in contraction for 20 掳C cooling of a lens 50 millimetres across would only be 0.05 millimetres. Assuming the lenses are properly located in the frame, this should not be sufficient to loosen the lens. I tested this by putting my reading glasses in the freezer 鈥� a 40 掳C cooling 鈥� with no noticeable effect. Some more experiments may reveal why the lenses fell out.
Philip Ward, Sheffield, South Yorkshire, UK