Students at the Massachusetts Institute of Technology created an optical illusion several years ago that also acts as a test to determine vision impairment. If you see Marilyn Monroe in the picture, you probably need glasses.
A healthy pair of eyes will see only Albert Einstein if they view the picture from a normal distance.
The illusion works by superimposing a lightly drawn picture of Albert Einstein over a blurry picture of Marilyn Monroe, showing that high spatial frequency features like the fine lines of Einstein are visible from up close, whereas the blur, low spatial frequency image of Monroe can only be seen from a distance.
Hence, a healthy pair of eyes can only see Monroe if seen from much further away (or seen as a thumbnail). Like this: