Physics
The mercury and sodium street lamps light up because of :

Electron absorption
Atomic emission
Electron emission
Atomic absorption

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Physics
Solids have definite shapes.This is because

The forces of adhesion are large
The forces of cohesion of the molecules within are very much less in them
The forces of adhesion are less
The forces of cohesion are very large

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Physics
Purpose of an optical filter is to

Transmit or absorb light of different colours
Dispense light into component colours
Refract light of different colours
Reflect lights of different colours

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