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

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

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Physics
Rainbow is due to

refraction and reflection of sunlight by water droplets
diffusion of sunlight through water droplets
ionisation of water deposits
absorption of sunlight in minute water droplets

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