Physics
Lightning is produced when

Strong opposite charges in different clouds break down the resistance offered by the intervening air
Water vapour produces electricity in the clouds
Similar charges of electricity rush towards each other and then get repelled
Clouds strike against impurities in air and the friction burns up these impurities

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Physics
Which of the following occurred first ?

Guglielmo Marconi sent out the first wireless signals
Max Planck unveiled the Quantum Theory
Wright Brothers successfully flew an aircraft
Albert Einstein propounded the General Theory of Relativity

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Physics
What is refractive index ?

It is defined as the ratio of speed of light in vacuum to the speed of light in the medium.
None of above
It is defined as the product of speed of light in medium and in vacuum.
It is defined as the ratio of speed of light in the medium to the speed of light in vacuum.

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