After closing my refrigerator's door I noticed that it's much harder to reopen it immediately, as if there's an underpressure. Only after a minute or so it opens normally. How can this be explained?
[Physics] Why does the refrigerator door resist opening
everyday-lifepressure
Related Solutions
You have created a rather poor pinhole camera (camera obscura). You can see an "image" of the sky, a green space (trees) and even a reddish brown blur that is your driveway.
This is not diffraction or refraction - it's geometrical (classical) optics. Because the hole is pretty big, you see a very blurry image. But basically, the light from the sky falling through the hole hits one part of the door; the light from the trees hits a slightly different part; and the light from the driveway (the lowest object on the outside) hits the highest part on the inside, after coming through the hole.
Incidentally - if you took a picture from the perspective of the hole in the door, I am guessing you would see more of the sky - in your photo of the view, the sky is barely visible, but in the doorhole image it's much more prominent; and conversely, one sees less of the driveway. This is consistent with the photo being taken from a different vantage point.
Tricky question (well a little bit tricky).
But the correct answer is - no change in pressure. Assuming the valve is closed very slowly so that it does not act to displace water as it closes, and that the valve creates a perfect seal as it is closed, the water in the container below is essentially trapped at the original pressure, $P=\rho g(H+h)$. When the valve is closed, the lower part of the container is no longer affected by the column of water above it, but it does retain the pressure from the original pressurization.
Best Answer
When you open the door the cold air flows out and is replaced by air at room temperature. When you close the door this air gets cooled through contact with the stuff inside the fridge and therefore the pressure decreases, according to ideal gas laws by some 10%. That's not very much, but the surface of the door translates this small pressure difference to a larger force.
Note that the effect is even stronger with deep freezers, where the difference in temperature (and hence pressure) is even bigger.
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Like Anna says there must be air leaking in if the door opens normally after some time. This air will be cooled as well, but the pressure will increase with more air being sucked in, until an equilibrium is reached: cold air at atmospheric pressure.