i am newbie. I have spent one day searching for solution and find nothing working...
Maybe i am searching for wrong keywords. I am sorry if this is stupid question.

Problem:
Photoshop 2017 CC
I am trying to re-create lenses for sunglasses and then put it on face.
I have cut out lenses from frame, changed color of lenses.
The goal is to make lenses (100% opacity)
with opacity continuously decreasing verticaly from 100% (upper part) to 40% (bottom part),
so bottom part will be semi-transparent and little of face will be visible via lenses.
I can not find settings which allow this.
I am able to change opacity of lenses only for whole lenses, not able to set decreasing opacity.

Any simple solution?
Thank You a lot!
Jenna