Retroreflector and railway semaphore: what they have in common

1. Step 1. Retroreflector — uses — Reflection of light. A corner-cube prism.
2. Step 2. Reflection of light — is used in — Otoscope. A small mirror directs the light.
3. Step 3. Otoscope — has part — LED. Illumination.
4. Step 4. LED — is part of — Traffic light. LEDs.
5. Step 5. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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