Amplitude is measured from the rest position to the crest (or to the trough), not from crest to trough. Measuring crest to trough gives twice the amplitude.
A wave with greater amplitude carries more energy. For sound, greater amplitude means louder. For light, greater amplitude means brighter (at the same frequency).
Amplitude is read from a displacement-time graph or a displacement-distance graph as the maximum displacement from the horizontal centre line.
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