A payment terminal creates a very local radio field
Tap-to-pay usually uses near-field communication, or NFC: a short-range wireless system built for devices held close together. Inside the reader, an antenna generates a changing electromagnetic field. When a card’s antenna enters that field, it can draw enough energy from it to respond and exchange payment data with the reader. A phone can communicate over the same kind of close-range link.
The antennas are designed to couple most effectively when they are only a small distance apart. Move the card away, and the signal reaching it drops quickly; the reader and card can no longer complete their exchange reliably. That is why a deliberate tap works while a nearby wallet generally does not.
The short range is one layer, not the whole safeguard
A tap is not simply a card broadcasting its number to everything nearby. The reader and payment device exchange information needed to process a transaction, and payment systems use additional security checks to help protect that exchange. Proximity makes it harder for an unrelated reader a few feet away to communicate with the card, but it is not a substitute for those other protections.
The practical effect is a narrow target: the card or phone must be brought close to the reader, often near its marked contactless symbol. In a crowded checkout line, that steep drop in signal strength helps keep a transaction tied to the device intentionally presented at the terminal.
