Light and dark are relative when you are speaking in terms of value and contrast. So the surface of a volume or can haverelative values of light and darkness.
Value and Contrast
Low contrasts in some images can be subtle messages however a high contrast is good for reading and can have a strong impact when used on certain compositions.
Value and SpaceWhen an image contains a large area of ethier end of value scale, it changes the overall feel of the image. Using values as atmospheric and ariel shading you can create space and volume with value.