Bending on the way in

The moment light enters water

When light passes from air into water, it changes direction. That is because light slows down in water, to about 225,000 km per second, about a quarter slower than in air. The dotted line stands straight out from the drop’s surface. Light that arrives 59° away from this line bends to 40° in the water, tilting toward the middle of the drop.

Why?Picture a row of students marching across a playground and walking into a sandpit at a slant. The students who reach the sand first slow down first, and the whole row swings around. Light does the same: the side that reaches the water first slows first, so the light turns.

Learn moreAbout 6% of the incoming light bounces straight off the surface (the faint line). The bending angle comes from the law of refraction (Snell’s law), and how much a material slows light is called its refractive index. For yellow light, water’s refractive index is about 1.333.

Bending angle

59° → 40°

Speed of light in water

about 224,840 km/s

Bounces off the surface

about 6%

calculated from the refractive index of water (IAPWS, 20 °C) and the laws of refraction and reflection