Lighting. This photograph shows three spheres with varying surfaces. The one on the left is matte, the one in the middle is glossy, and the one on the right is highly reflective. On the matte sphere there is no highlight. On the glossy sphere in the middle, the highlight is clearly apparent. The mirror-like sphere on the right also has a highlight, which is really a reflection of the light source. The light source is the sun shining through a window. The right sphere also reflects the scene around the ball, including the white paper background and the dark room behind the camera.
That same pattern of reflections of the paper, room, and neighboring ball is subtly visible in the middle ball as well. In the middle ball, there is a second, smaller highlight just to the right of the primary highlight. So we've arrived at a definition: A highlight is a specular reflection (Latin "speculum"=mirror) of the light source on a shiny surface. These three diagrams show what's happening at the surface level. Mall. A u d r e y * k a w a s a k i. My Painting a Day Blog.