This article investigates the optics misconceptions of 220 year 11 Thai high-school students. These misconceptions became apparent when the students attempted to explain how an object submerged in a water tank is "seen" by an observer looking into the tank from above and at an angle. The two diagnostic questions used in the study probe the students' ability to use a ray diagram to explain the relationship between object, image and observer, and then to use the ray diagram to qualitatively determine the position of the image. The study indicates that these high-school students, even after instruction, had significant misconceptions about the direction of propagation of light, how light refracts at an interface, and how to determine the position of an image. The study revealed that students used various concept models to explain how the object can be "seen" in this situation. Only 22% of all students had a qualitative understanding of how to use a ray diagram to determine image position, and only 1 of 220 students could identify the correct image position using correct reasoning. Our results indicate that students require very careful instruction if they are to understand how objects are "seen" and how images are formed when light refracts through a planar surface. (Contains 7 figures and 4 tables.)