Physics
A soap bubble shows colours when illuminated with white light. This is due to

Diffraction
Polarisation
Reflection
Interference

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Physics
What is refractive index ?

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

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Physics
When a piece of pure silicon is doped with aluminium, then

The doped silicon piece will become p-type
The resistivity of the doped silicon piece will increase
The conductivity of the doped silicon piece will remain the same
The doped silicon piece will become n-type

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