Light rays are bent upon moving between media of different indices of refraction. Additionally, light rays can be reflected from surfaces. These two properties of light propagation cause images to appear distorted from their original form. This shape change can come in the form of enlargement or reduction in size (magnification), inversion, and even anisotropic stretching. The changes in images due to the bending of light rays allow you to view distant and microscopic objects, which otherwise could not be seen. (Thats why lenses were so important in the development of modern science!)
