How does the barcode reader works?

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On payment counters at malls the barcode reader is used to read the barcode on item.The barcode reader reads the barcode only when the rays from the barcode reader fall on the code.I have heard that if an item is not submitted at the counter and is not read by the barcode reader then there will be an alaram when the item is going out of the mall.How does the alaram detects that the item is not read by the barcode reader.