Rebound and Refract

Rebound went first, to show the bouncing-back.

She clicked a sound down at the water's surface, and part of it came echoing right back up to her ears. "This is the REFLECTION half," she said. "The part of the wave that hits the boundary and bounces back the way it came — an echo." She clicked again, timing the return. "It's how I fly in the dark, how sonar finds the sea floor, how you see yourself in a window or a still pond." Ripple watched the sound bounce back up. "So the wave bounced off the surface," Ripple said. "Part of it did," Rebound agreed. Then she paused. "But listen — not ALL of my click came back. Some of it kept going, DOWN into the water. My echo only tells me about the part that bounced. Where did the rest go?" She looked down through the surface. "For the part that went THROUGH — the part that bent onward into the water — I need her."

Ripple considered. "So YOUR half is the part that bounces back... but some of the wave keeps going and you can't follow it."

"Only the bounce," Rebound agreed. "Half of what happens at a boundary. The other half is hers."

Then Refract took over, to show the bending-through.

She caught the part of the wave that had crossed INTO the water and bent. "This is the REFRACTION half," she said. "The part of the wave that DIDN'T bounce back — that crossed into the new stuff and bent, because it changed speed." She traced its bent path downward. "It's why a straw looks broken, why I can see the bent world above the water, why a lens can focus light — the part that goes through, bending." Ripple watched the light bend down into the tank. "So that part went through and bent," Ripple said. "It did," Refract agreed. Then she looked up at the surface. "But notice — the part I follow is only the part that crossed. Some of the wave never came down to me at all; it bounced back up to Rebound. My bent ray tells me nothing about THAT part." She glanced up through the surface. "For the part that bounced back, I need her."

Ripple looked from the bat above to the fish below. "So HER half is the part that bounces back... and YOUR half is the part that bends through. And at every boundary, BOTH happen — the same wave splits into a bounced part and a bent part."

"Two halves of one boundary," Refract said. "Every time."

Then Rebound and Refract studied one wave hitting the surface TOGETHER, and Ripple finally saw the boundary split it in two.

A single wave came down and struck the water's surface. And right there, at the boundary, it split: part of it BOUNCED back up to Rebound (the reflection), AND part of it BENT down into the water to Refract (the refraction) — both at the very same instant, from the very same wave. Ripple looked at the bounced part going up and the bent part going down and understood: a boundary doesn't make a wave choose. It splits the wave into a reflected part and a refracted part, together.

"OH," Ripple breathed. "The SAME wave both bounces back AND bends through — that's why a window is a mirror AND a window at once!"

"That's it," Rebound said. Alone, her echo only knew the bounced part. Alone, Refract's bent ray only knew the part that crossed. But together — reflection AND refraction — they described the WHOLE of what a wave does at a boundary. "Part bounces back to me," Rebound said. "And part bends onward to me," Refract said. "Both at once," they said together. "A boundary splits every wave into a bounced part and a bent part — reflection and refraction, the two halves of one event."

Ripple tested it: he shone a light at the tank's surface at an angle, and saw a faint reflection bounce off the top AND a bent beam continue down into the water — both from his one light, at once. "It reflected AND refracted," Ripple said, "from the same beam, at the same boundary."