Physics
Light Emitting Diodes (LED) is used in fancy electronic devices such as toys emit

ultraviolet light
visible light
X-rays
radio waves

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Physics
Conservation of energy refers to the fact that

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

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Physics
Dynamo is a device for converting

Magnetic energy into electrical energy
Mechanical energy into electrical energy
Chemical energy into electrical energy
Heat energy into electrical energy

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