The whole screen will be clear when you drag the scanning eye. You can draw any pattern you like, and the java program will draw the virtual image of it. When you draw in the water region, the region where the total internal reflection is possible, will be shown. If you click the right mouse button, it will show you many light rays emitted. Right click the mouse button twice is another way to clear the screen.
Light emitted from the object is refracted at the water and air interface.
The observer trace back those refracted light to find the image of the object. The image appear closer to the water surface and its shape is different from the object. The world viewed under water is different from what we view it above the water surface.