A wrong answer from my IELTS practice today enticed me, a layman of art to wade through the scientific papers about the idealism and realism in art. Here is my findings:
Idealism artists believe that there is a limitation of what can be seen naturally, therefore, they perfect and embellish what they behold. One good example is the Distant View of Niagara Falls in 1830. The image represents little of actual landscape near the falls at the time, which was marked by factories and hotels.
On the other hand, Realism in art usually means copying or imitation of objects and scenes. For instance, Portrait of an Old Woman, by a German artist of the 18th century Balthasar Denner, pays special attentions to the details of the face. He aimed to copy every wrinkle on the face as if it was photographed.