What is an example of imagery creating mood in this story?

The very last image is a quietly spooky one: having gathered courage after the knocking stopped, the Whites look out their door and see nothing but a street lamp flickering on a deserted road. All is silent, and there is no trace of their dead son. This image calls into question everything that has just happened, or not happened. Was it really Herbert? Was there really knocking at all? If it was Herbert, where is he now? What exactly did Mr. White wish for?