Physics
A light ray travelling from denser to rarer medium at an angle of incidence more than critical angle of concerned media pair suffers.

Total internal reflection
Refraction
Reflection
Diffraction

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Physics
Siphon will fail to work if:

Both its limbs are of unequal length.
The level of the liquid in the two vessels are at the same height.
The densities of the liquid in the two vessels are equal.
The temperature of the liquids in the two vessels are the same.

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