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I was reading about planet Jupiter and found that it can cast shadow of an object on Earth though faint

My question is how many photons /unit area are required for any light source to cast shadow of an object?

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    $\begingroup$ Depends on how accurately you want the shadow's borders to be defined. $\endgroup$ Jul 31, 2017 at 3:25

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This double slit experiment one photon at a time gives an order of magnitude on the number of photons needed for light to appear with its usual interference protperties:

sinlsphotondbl

Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

The classical light fringes start being detectable at by the second frame. The first frame has order of 100 photons, the second 100,000.

I would guess that for a small dimensional object, order of cm^2 a shadow would appear at the hundreds of thousands of photons.

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