The physics of how a laser works is complicated. A laser emits a very narrow beam of light. This light hits the bar code and is then reflected back towards the scanner. The amount of light that is reflected back to the scanner is greater if the surface is white and the amount reflected back is less if the surface is black. As a laser beam passes over a bar code it makes a pattern of reflected light. The scanner has a sensor that can tell how much light is being reflected. The scanner takes the information from the scanner and sends it to a computer. If the computer has the right software it can identify what product the code refers to and what the price of that product is.