Physics
Light travels in a straight line (rectilinear propagation of light). This statement does hold if the medium of travel for light is

Homogeneous and transparent
Made up of slabs of different refractive indices
Inhomogeneous and transparent
Of variable refractive index

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Physics
Siphon will fail to work if:

The temperature of the liquids in the two vessels are the same.
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.
Both its limbs are of unequal length.

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