Understanding the mechanism of different imaging characteristics is necessary for image interpretation and information extraction of internal solitary waves (ISWs). In this article, the experimental method is used to reveal the mechanism of different bright and dark patterns in optical remote-sensing images. It provides a scientific method to interpret the imaging characteristics of ISWs in optical remote-sensing images. The results prove that there are two critical angles which are important for optical remote-sensing image interpretation. The critical angle is related to the zenith angle of the light source, sensor, and the wave slope modulated by ISWs. When the zenith angle of the light source is fixed and the zenith angle of the sensor is located between two critical angles, the ISWs appear as dark-bright pattern in optical remote-sensing images. Otherwise, the ISWs appear as bright-dark pattern. This method has been applied to the images acquired around the Dongsha Islands in the South China Sea, and similar results are obtained which indicate that the method is effective and applicable.