Physics
The sky appears blue because

The atmosphere scatters blue colour more than the others
In white light, the blue component dominates
All colours interfere to produce blue
It is actually blue

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Physics
Holography is a technique of

Recording a permanent three dimensional black and white photograph
Recording a permanent, sharp two diimensional black and white or multicolour photograph
Recording a permanent three dimensional photograph of a given single colour or a multicolour
Recording a permanent three dimensional multicolour photograph

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