Barcodes and QR Codes: How They Work and Where They Are Used

At a supermarket checkout, the cashier passes each item over a red light, there's a beep, and the name and price appear on the screen. At a church event or a concert, someone scans a square pattern on your phone and lets you in. Both rely on codes made of patterns that a machine can read in a split second. This note explains how barcodes and QR codes work.

Year 2: Barcodes and QR codes

What a barcode is

Look at the back of a packet of biscuits and you'll see black bars of different widths with a number underneath. Those bars are just another way of writing that number, one a scanner can read instantly.

The textbook definition is: a barcode is a series of lines of varying widths that represent numbers and other symbols. Each barcode is a globally unique number, so no two different products share one.

Here is what happens when an item is scanned. The barcode scanner, an input device, shines light on the bars and reads the pattern as a number. The computer looks up that number in the shop's database to find the product's name and price, and the result is displayed on the screen and printed on the receipt, both output devices. Because every sale is recorded, the shop owner can see which items sell most, which sell slowly and which are out of stock, and can track goods through the supply chain. Libraries use the same idea: scanning a book's barcode shows its title and author.

Types of barcodes