[{"_id":1,"semester":1,"grades":[70,87,90]},{"_id":2,"semester":1,"grades":[90,88,92]},{"_id":3,"semester":1,"grades":[85,100,90]},{"_id":4,"semester":2,"grades":[79,85,80]},{"_id":5,"semester":2,"grades":[88,88,92]},{"_id":6,"semester":2,"grades":[95,90,96]}]

db.collection.find({semester:1,grades:{$gte:85}},{"grades":{$filter:{input:"$grades",cond:{$gte:["$$this",85]}}}})