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Shadows and reflections are the effects of light. Shadows are formed when objects block a source of light. Night is caused by the Earth blocking the light from the Sun - on the half of the Earth in shadow it is night time. Even at night time you can usually see some light - maybe from streetlamps, the moon, the stars or the headlamps of passing cars. Have you ever been anywhere so dark that you couldn't see your hand in front of your face? Reflections are caused by light bouncing off objects. Smooth things usually make better reflectors than rough ones. Mirrors are very smooth and they are the best reflectors.
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