The barcode scanning gun used in supermarkets is based on the principle of optical scanning. 

Barcodes are a series of vertical bars and spaces of varying widths that represent a unique code for a product. The barcode scanning gun uses a light source, typically a laser or LED, to illuminate the barcode. 

The light reflects off the barcode and is detected by a photosensitive sensor in the scanner. 

The sensor converts the light into an electrical signal, which is then decoded by the scanner’s software to retrieve the product information. 

This information is then sent to the supermarket’s point-of-sale system, which records the sale and updates the inventory. 

The barcode scanning technology has revolutionized the retail industry by making the checkout process faster and more accurate.