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If you defocus your eyes and look at a source of light – say a street lamp – and then slowly move an opaque object in front of it, the light disappears 'backwards'. (Note: this effect is most apparent when looking at something relatively small in your field of view.)

For example, if moving an opaque object upwards so that it touches the light source (as perceived from your viewpoint) from the bottom first, the top part of the light source is obscured first, and then as you continue to move the opaque object up, more and more of the light source is obscured until it is entirely out of sight. It looks as if in fact an object has come down from above rather than up from below to hide the light from your eye.

Why is this?

Josh
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  • Possibly related question? How does light bend around my finger tip? – BioPhysicist Jul 29 '19 at 21:46
  • I think the answer is indeed in that post, where Ilmari states "The direction in which the blurred image shifts depends on whether the lens is focused a little bit too close or a little bit too far. If the focus is too close, as in the diagrams above, the image will appear shifted away from the obstacle. (Remember that the lens inverts the image, so the image of the obstacle itself would appear above the image of the dot in the diagram!) Conversely, if the focus is too far, the background object will appear to shift closer to the obstacle." – JMLCarter Jul 29 '19 at 21:55
  • It will depend on whether you are myopic or hyperopic. The myopic viewer will in general focus in front of an object, the hyperopic behind. If astigmatic it is possible to do both in different planes. A quick test is to look at the speckle from a laser and move your head left/right and up/down to see whether the speckle moves in the direction of motion or opposite to it. – Paul Childs Jul 30 '19 at 01:24
  • @PaulChilds Comments are not for answering questions. – BioPhysicist Jul 30 '19 at 11:17

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