Radio Frequency Identification (RFID) is a wireless technology that lets devices detect objects with special tags embedded in them. The tagged objects need to be near the RFID reader. For instance, many train stations use it to sense the microchip in your transport card. The gate lets you through even when the card is in your wallet or purse because it has identified it using RFID.
In a way, it’s like bringing a magnet near some iron shavings. When brought close enough, the iron detects the presence of the magnet and starts moving toward it.
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