Is NFC a radio wave or a magnetic field?
NFC uses a magnetic field, not a propagating radio wave. The reader and the tag sit so close together that they are coupled like two windings of a transformer, exchanging energy rather than radiating it. This is what the phrase near field means, and it is meant literally rather than as a loose way of saying short range.
The difference between coupling and radiating
Close to an antenna, energy sloshes back and forth between the antenna and the space around it without leaving. Further out, part of that energy detaches and propagates as a wave that keeps going whether anything receives it or not.
NFC lives entirely in the first regime. The reader is not broadcasting and hoping something hears. It maintains a field, and a tag placed inside that field draws from it. Pull the tag out and the energy stays with the reader.
Where the boundary sits
At 13.56 MHz the wavelength is about 22 metres. The near field extends roughly to the wavelength divided by two pi, which is around 3.5 metres.
A tap happens at two centimetres. That is not near the boundary, it is more than a hundred times inside it. Nothing about an NFC exchange is remotely close to radiative behaviour.
Why the name is precise
Near field communication is one of the rare technology names that says exactly what the thing does. It is not marketing shorthand for short range. It names the physical region the exchange takes place in, and everything characteristic about NFC follows from operating there: the tiny range, the ability to power a passive object, the immunity to being read from across a room.
Why shielding behaves oddly
Blocking a propagating wave and blocking a magnetic field are different problems, which is why intuitions from Wi-Fi mislead here.
A thin conductive sheet does not simply absorb the field: it develops circulating currents that oppose it and detune the tag antenna. The result is usually a complete failure rather than a weakened read, and it happens at a distance where a wave would barely be attenuated.
Conversely, materials that would obstruct a higher frequency signal often make no difference at all. Wood, plastic, glass and leather are effectively invisible to a 13.56 MHz magnetic field.
What follows
Nobody can read your tag from across the room, and no reader can be made powerful enough to change that, because the limitation is geometric rather than a matter of transmitted power.
Wrapping a card in foil is popular advice, and it works, but for a reason people usually get wrong: the foil is not a barrier the field cannot cross, it is a conductor that spoils the coupling. Which also means an imperfect wrap can behave unpredictably rather than simply working less well.
Frequently asked questions
Does NFC emit radiation?
It produces an oscillating magnetic field, which is non ionising, at power levels far below a mobile phone call. Whether that raises any health question is a separate topic with its own literature, and it is covered in the security and privacy section rather than here.
Can NFC be picked up by a radio receiver?
Not in any useful way at normal distances. There is no propagating signal to receive, only a local field. Eavesdropping on an exchange requires specialised equipment held very close, which is a different threat from the passive interception people usually imagine.
Why does an RFID blocking wallet work if there is no wave to block?
Because a conductive layer disturbs the coupling rather than absorbing a wave. The card and the reader can no longer exchange energy efficiently, so the read fails. The name describes the outcome, not the mechanism.
Does the field pass through the human body?
Partly, but tissue is conductive enough to absorb energy and detune an antenna, so a tag pressed between a phone and a hand often reads worse than one on a table. This is one reason a card in a back pocket behaves differently from the same card on a desk.