I don't believe my eyes!

Michael has found an old pair of glasses in the cellar of his grandparents. He knows that his grandmother needs glasses to be able to see distant objects. His grandfather, on the other hand, needs his glasses for reading and to be able to see close objects. Michael would like to find out whose glasses he had found in the cellar. He could always ask, but that would be so boring. Instead Michael takes a closer look at the glasses. He realizes that the glasses are thicker in the middle compared to its edges.
What can you conclude about the function of the glasses?

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