This is a really easy trick to give you a gradual fade on any object in Illustrator. This works well for reflections. First, make your object and make a copy, and go to object>Reflect>horizontal. This will flip the copy, to where it is upside down. Position your objects as shown.
Then, create a rectangle the size of the bottom object. give it a gradient fill, using white to black. This works just like any normal mask, where black conceals and white reveals.
Put the gradient rectangle over the part that you want to mask. Select the rectangle and the object you ant to mask, then go to the transparency menu, click the flyout menu and select make opacity mask.
