Physics
The electrical domestic tube light gives white fluorescent light because of

Colision between molecules of filled gas under electric current
Heavy current
Vacuum inside the tube
Falling of ultra violet rays on the white inner coating of the tube

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Physics
Conservation of energy refers to the fact that

If you conserve energy you will not be as tired
Energy cannot be created or destroyed but can be converted from one form to another
The entropy of the universe is always increasing
No chemical reaction is 100 percent efficient

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