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When the miners get out of the mine after work, they can’t adapt to the sudden change of brightness in a short time and the strong light may harm their eyes. This pair of glasses is made of a sort of optical glass which contains silver halide micro crystal. The color of the glass becomes lighter as it contacts sunshine. It’s a pair of goggles when they work underground in the mine. Once they get out of the mine, this pair of glasses gradually becomes transparent to protect their eyes.