The X-rays are very penetrative rays having much shorter wavelength than that of the visible light. So they are able to penetrate some objects which are opaque for the visible light. E.g. when X-ray photograph of a patient is taken these rays fully penetrate the soft muscles, are somewhat absorbed in highly dense muscles and are absorbed to a great extent in hard boned. The image in X-ray photograph shows the skin and soft muscles as completely black portion through which X-rays have penetrated and as white portion those parts of the body in which X-rays have been absorbed.