To show how the use of light affects a photograph, here are two images, virtually identical apart from one the first image has been lit so the models face is fully in view, the second, so that part of her face is in shadow.
The use of light here affects how we view the subject. The second image looks much more serious than the first, nothing has been changed about the image, same model, same clothes, same facial expression. But the use of shadow across the face gives us a much more dramatic effect than the first image.
This is effect is achieved with the use of two studio lights. The first is set to a bright setting, lighting the subject from one side, the other is set very low, meaning that there is enough light to show the face, but not bright enough so it caused a shadow to fall on that side of the models face.