Light travels fastest in:

**The reasoning** Light is an electromagnetic wave that travels at different speeds through different materials. The key principle here is **optical density and refractive index**. In a vacuum (empty space with no particles), light travels at its maximum possible speed: approximately 3 × 10⁸ m/s (300,000 km/s). This is the universal speed limit — nothing goes faster! When light enters any material medium (water, glass, diamond), it interacts with the atoms and molecules there. These interactions slow it down. The denser the material's atomic structure, the more light gets slowed. - Vacuum: 3.0 × 10⁸ m/s (fastest) - Water: about 2.25 × 10⁸ m/s - Glass: about 2.0 × 10⁸ m/s - Diamond: about 1.24 × 10⁸ m/s (slowest here) **Why the wrong options tempt you** Students sometimes think "solid = stronger = faster" or confuse hardness with speed. Diamond is the hardest substance, but that actually means its atoms are tightly packed — which slows light down more! Water and glass feel "transparent," so you might assume light zips through easily, but particles still create resistance. **Quick takeaway** Light sprints fastest where there's nothing to slow it down — in the emptiness of a vacuum.