Mongo DB: Get all documents inserted after the last known one

> db.events.find()
{ "_id" : ObjectId("50911c3e09913b2c643f1215"), "context" : "jvc8irfjc9cdnf93", "key" : "value" }
{ "_id" : ObjectId("50911c4709913b2c643f1216"), "context" : "jvc8irfjc9cdnf93", "key" : "new value" }
{ "_id" : ObjectId("50911c4b09913b2c643f1217"), "context" : "jvc8irfjc9cdnf93", "key" : "newer value" }
{ "_id" : ObjectId("50911c4f09913b2c643f1218"), "context" : "jvc8irfjc9cdnf93", "key" : "newest value" }
{ "_id" : ObjectId("50911c6309913b2c643f1219"), "context" : "0djd8vcndkfnjhv3", "key" : "value" }

is a list of events on a Mongo DB server. The client keeps track of the last event he knows about, for example 50911c4709913b2c643f1216, the second one.

How to I get all events that have been inserted after this known one? In this case 50911c4b09913b2c643f1217, 50911c4f09913b2c643f1218 and 50911c6309913b2c643f1219.