To change sentences from active to passive voice, we need to follow a few steps. Let's go through each sentence one by one.

1. People built the Tower of London in the 11th century.
   • Identify the object: "the Tower of London".
   • Identify the verb: "built".
   • Change the subject to the object and the object to the subject. The new subject will be "The Tower of London".
   • Use the appropriate form of the verb "to be" (in this case, "was") and the past participle of "build" (which is "built").
   • Rearrange the sentence: "The Tower of London was built by people in the 11th century."
2. Tourists visit Buckingham Palace every summer.
   • Object: "Buckingham Palace".
   • Verb: "visit".
   • Change the subject to the object: "Buckingham Palace".
   • Use the appropriate form of "to be" (present simple: "is") and the past participle of "visit" (which is "visited").
   • Rearrange: "Buckingham Palace is visited by tourists every summer."
3. They opened the British Museum in 1759.
   • Object: "the British Museum".
   • Verb: "opened".
   • Change the subject to the object: "The British Museum".
   • Use the appropriate form of "to be" (past simple: "was") and the past participle of "open" (which is "opened").
   • Rearrange: "The British Museum was opened by them in 1759."
4. The students have taken photos of Westminster Abbey.
   • Object: "photos".
   • Verb: "have taken".
   • Change the subject to the object: "Photos".
   • Use the appropriate form of "to be" (present perfect: "have been") and the past participle of "take" (which is "taken").
   • Rearrange: "Photos of Westminster Abbey have been taken by the students."
5. They will build a new bridge across the Thames.
   • Object: "a new bridge".
   • Verb: "will build".
   • Change the subject to the object: "A new bridge".
   • Use the appropriate form of "to be" (future simple: "will be") and the past participle of "build" (which is "built").
   • Rearrange: "A new bridge will be built by them across the Thames."

Now we have transformed all the sentences into passive voice. Remember, in passive constructions, the focus shifts from who is doing the action to the action itself and the recipient of the action.