Blue LED and optical telegraph: what they have in common

1. Step 1. Blue LED — is a kind of — LED. A blue LED.
2. Step 2. LED — is part of — Traffic light. LEDs.
3. Step 3. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.
4. Step 4. Railway semaphore — evolved from — Optical telegraph. The semaphore idea.

Open Evothings