What is the importance of light and shade to photography?

Light and shadow are particularly important for photography. Because it is black and white and countless shades of gray levels or various colors in color photography that form the visible image in the photo. Based on the different feelings brought by different shades of color in life practice, such as the clear sky makes people feel comfortable and the dark clouds make people feel depressed and sad, photography always skillfully uses the changes of light and shade to make the works more strongly infect and impress the audience.