A translator gets distracted by every rustle in the room, and a one-hour meeting stretches into two. In the same way, gravitational waves passing through empty space can't give rise to light: the laws of nature forbid a photon from turning into a graviton. But if you put light inside a metal box, the walls change the rules. Inside such a box, a gravitational wave can set a pair of photons swinging, the way a strong gust of wind sets a swing in motion.
When the gravitational field is treated as classical, the number of photons grows very slowly. For realistic parameters, fewer than one photon is born in 2 seconds. But if you take into account that gravity is quantum too, the picture changes. Gravitons are the quanta of the gravitational field, just as photons are the quanta of light. Gravitons give up energy to photons and weaken themselves, so the growth of photons saturates. The photons become entangled with the gravitons, and this can be measured. If the purity of the photon state drops, then gravity really is quantum. That's how a box with metal walls becomes a device for testing the fundamental laws of nature.
At a typical gravitational wave strain of 10⁻²¹ and a frequency of 3.9 GHz, a cavity holds about 10²⁴ gravitons, yet generating a single photon would take 10¹² seconds, which exceeds the age of the Universe.
The idea of a cavity that converts gravitational waves into light echoes the concept of "gravitational radio" from science fiction, for instance in Carl Sagan's novel Contact, where an alien signal uses gravitational waves for communication.