灯
ひ · hi · JLPT N3 · noun
light
🏮 灯 = 火 fire + 丁; read hi just like 火, because fire was the first light
The left side of 灯 is 火, fire, which is also read ひ (hi). Before electricity, every lamp was a small flame, so the word for light shares its sound with fire. The right part, 丁, mainly tells you the Chinese-style reading, とう (tō).
灯 — fire radical 火 + ding 丁 (phonetic)
灯 is a semantic-phonetic compound: 火 'fire' gives the meaning (lamps once burned oil or flame), while 丁 hints at the sound. It is the simplified form of the traditional 燈, which used 登 as the phonetic. Today it means 'lamp' or 'light'.