Alternative text (alt text) is meant for readers who cannot see an image, such as blind readers and readers who use a text or mobile browser. It should summarize an image's purpose, and should not duplicate its caption. Every image should have alt text, except for purely decorative images, which should instead have "|alt=|link=".
Nearly free electron model
The following table shows images and captions on the left, and alt text and captions on the right: the right column is what a visually impaired reader will hear. This table was computed from the copy of Nearly free electron model cached on 03 May 2013 at 15:08.
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 | Wikipedia book
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Some free electron bands in a FCC crystal structure according to the Empty Lattice Approximation
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