Uploaded November 2013 | Updated September 2026, 2 weeks ago

---

Shining a laser through tap water doesn't do much.
Shining a laser through tonic water does.

Laser Writing video: youtu.be/Xb1T87Vz4Xk

The Physics:
Normally it's only possible to see a laser beam if it's reflecting off something. Smoke machines fill the air with particles of smoke that laser light can bounce off - allowing beam to show up. Normal water doesn't reflect much laser light so it's difficult to see the laser beam through water.

Tonic water water is doing something different. Quinine molecules in the tonic water absorb laser light and then re-emit it. It's as though the tonic water itself is glowing. Quinine has a "quantum yield" of about 0.62 which means that for every 100 photons absorbed, 62 will be re-emitted. As a result, quinine glows quite fiercely when struck by the correct wavelength of light.