Deception is used in research in order prevent participants from behaving in an expected way. For example, if participants know they are being observed about how angry something makes them, they may behave with more anger since that is what the study is about. Or they may want to seem as though they do not easily anger and try to show no anger at all.
Deception can be a helpful tool in conducting experiments. Discuss when and how much deception is acceptable and when a researcher has taken deception too far.