Laser and gas lamp: what they have in common

1. Step 1. Laser — is made of — Semiconductor. Maiman’s first laser was a ruby crystal; today it is more often a semiconductor.
2. Step 2. Semiconductor — is material for — Photoresistor. Selenium and cadmium sulfide are semiconductors.
3. Step 3. Photoresistor — is part of — Street light. Switching on at dusk.
4. Step 4. Street light — evolved from — Gas lamp. Gas lamps.

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