Physics
Fiber optics cable used in communication, works on the principle of

Refraction of light
Diffuse reflection of light
Regular reflection of light
Total internal reflection of light

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Physics
It is easier to roll a barrel full of coal tar than to pull it because

coal tar is a liquid and it flows in the barrel when it is rolled
the full weight of the barrel comes into play when it is pulled
the rolling friction is much less than the sliding friction
more surface area of the barrel is in contact with the road when it is pulled

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