What is the ruling on praying in front of a glass door with a curtain, if there is light behind the door that causes the shadow of the person knocking to appear?

The passing of something in front of a worshipper does not invalidate their prayer, but it diminishes its reward. The passing of a person behind a sutrah (barrier) and the presence of a shadow in front of the worshipper do not affect the prayer. This is based on the saying of the Messenger of Allah, peace and blessings be upon him: "Nothing severs the prayer, and ward off as much as you can."