Physics
After long periods of use, a grey spot develops on the inside of a bulb. This is because

Dust inside the bulb condenses on the top
The heat of the bulb scorches the glass at the top
The tungsten filament evaporates and collects there
Glass undergoes a change due to the heat

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Physics
Planets do not twinkle because

They are very far away from the earth resulting in decrease in intensity of light
They emit light of a constant intensity
Their distance from the earth does not change with time
They are nearer to earth and hence we receive a greater amount of light.

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Physics
Why are we able to hear short wave broadcasts better than long wave broadcasts ?

Short waves are more energetic than long waves.
Short wave broadcasts are made by nearby radio stations.
By convention, short waves are meant for long distances, whereas long waves are reserved for short distances.
Short waves are unaffected by atmospheric disturbances.

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