Optical disc (CD/DVD) and tunnelling shield: what they have in common

1. Step 1. Optical disc (CD/DVD) — uses — Laser diode. Read by a laser.
2. Step 2. Laser diode — uses — P–n junction. A p–n junction emits the light.
3. Step 3. P–n junction — is used in — IGBT. A transistor structure with p–n junctions.
4. Step 4. IGBT — is part of — Electric locomotive. Traction converters.
5. Step 5. Electric locomotive — is used in — Metro. The first lines were steam-operated; electric trains since 1890.
6. Step 6. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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