Glide Reflections

Remember a glide reflection g involve the combination of a reflection in a mirror with a translation parallel to that mirror. But then the composition of g with itself (g²) will be a translation. By the above though, the translation distance must be a whole number of units ....

Mirrors:

Where can this mirror be if the strip $-1 \leq y \leq 1$ is to be transformed to itself?

Translation Part:

How many units can the translation part of the glide reflection be?