In a snail race, you time how fast your snail is slithering.

In a snail race, you time how fast your snail is slithering. It is slow so he has devised a scheme to use interference of light to help him measure it. Two parallel thin glass plates are oriented vertically as seen below. The plate on the right moves to the right at a constant speed "v". When 500 nm light is incident normal to the plates from the left you observe the light to be strongly reflected every 10 seconds. What is the speed of the glass plate on the right?