World 1 · Reflection & Refraction · 7 minRefractive Index
Light slows down in matter
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Light moves fastest in vacuum (c ≈ 300,000 km/s). In matter it slows. The refractive index n compares the two speeds: n = c / v.
Typical values: air ≈ 1.00, water ≈ 1.33, crown glass ≈ 1.52, diamond ≈ 2.42. A larger n means light crawls more slowly and bends more sharply when it enters.
Check yourself
Light travels slowest in which of these?
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