First, remember that imagery is descriptive language that appeals to one of the five senses (sight, sound, taste, touch, smell). The Scarlet Letter is full of visual imagery, particularly in the areas of color, light and dark, and nature.
I encourage you to use the light and dark imagery to explore the honesty versus secrecy theme that is so prevalent in this novel. One of the best scenes to find examples comes in Chapters 16-19, when Hester and Pearl take a walk in the forrest and find Dimmesdale there. Notice as you re-read these chapters the use of shadow and sunlight imagery. Often, Pearl is seen only in sunlight ("Pearl had stood still in the sunshine" p.169), while the clouds tend to put shadows only on Hester.
In this scene, Pearl asks Hester about "The Black Man." This term could be a representation of the devil, or simply the personification of evil. Note that it is only in the secrecy (shadows) of the forrest that Hester feels safe to discuss that she has actually met the Black Man once in her life.
Later in the scene, Dimmesdale walks by, and he and Hester are able, for the first time since the first scaffold scene, to regard one another without the judgmental eyes of the townspeople. Hester has the courage to momentarily remove her scarlet letter and when she does, it is as if the shadows and gloom of the forrest momentarily lift, and sunshine fills the space as well as her heart.
Essentially, these four chapters use sunlight and shadow as images which parallel honesty and secrecy, which is a deeper level of a good versus evil theme. This is also a point of transformation for Hester, one in which she allows herself to be honest out loud for the first time in the story. I think you will find several examples within this forrest scene to make several points on this theme.