Adverb
specularly (comparative more specularly, superlative most specularly)
In a specular manner (in the manner of a mirror)
Because light specularly reflected from water does not usually reach the viewer, water is usually considered to have a very low albedo in spite of its high reflectivity at high angles of incident light. Source: Internet
In the picture on the left, the horizontally oriented polarizer preferentially transmits those reflections; rotating the polarizer by 90° (right) as one would view using polarized sunglasses blocks almost all specularly reflected sunlight. Source: Internet