Place the chopped cucumbers in a blender. Blend until smooth, then strain the pulp through a fine-mesh sieve to collect about 2 cups of cucumber juice. You can also use a juicer if you have one.

In a pitcher, combine the cucumber juice, water, lemon juice, and simple syrup.

Chill the lemonade for at least 30 minutes, then serve over ice with optional cucumber and lemon slices.