Luke likely found out during the events of Return of the Jedi , when he was with the Emperor on the Death Star and Yoda had already died off-screen earlier in the story.

What that means

  • In the actual films, Luke never sees the Emperor and Yoda’s duel directly, because that fight happened earlier in Revenge of the Sith.
  • The best-supported reading is that Luke learns about it by the time of the Emperor’s confrontation with him in Return of the Jedi.
  • Some fan discussion and novelization commentary also point to the Emperor realizing Yoda had died around that same period.

Simple timeline

  1. Yoda fights Palpatine in Revenge of the Sith.
  2. Yoda goes into hiding and later dies before Return of the Jedi.
  3. Luke becomes aware of the broader Sith/Jedi history during his encounter with the Emperor.

In plain English

So, Luke didn’t “find out” in a dedicated scene; he most likely learned it through the larger story background by the time he faced Palpatine in Return of the Jedi.