Figure 7

A sidewalk experiment. Different types of specular reflection as viewed on a sidewalk on the University of Idaho campus. (A) Solid surfaces can produce specular reflections if their surfaces are smooth at relevant wavelength provided that they are oriented in the correct direction to specularly reflect the Sun into the observer’s direction. In this case, each little arrow points to a bright speckle coming from a smooth piece of gravel sticking out of the concrete. (B) Here water has been dumped onto the surface. The water in this case follows the rough contours of the concrete, but the water’s surface tension keeps it smooth at visible wavelengths and the index of refraction allows for a strong specular reflectivity. Hence there is a specular reflection, but a broad one due to some angled facets having specular geometry even away from the central specular point. (C) This image shows a specular reflection from perfectly smooth water inside the first-author’s Kepler coffee mug. The specular signal is concentrated at a single point, and is particularly bright when compared to the weak signal from the solid surface in A.