Reflection Type and Angle
When you look at a not so smooth surface such as your
counter top in a classroom, at an oblique angle you see reflections of
objects; but you do not see reflections (or as clearly) when you look
vertically down. Why?
This situation is described by the Fresnel equations, which are studied in
advanced optics classes. When light is incident normally on an air-glass
interface, about 4% is reflected; this number depends only the index of
refraction of the glass and the air. When light is incident at a glancing
angle on glass or almost any dielectric surface (even the page of a book),
it is almost 100% reflected. So if you look at a light reflected off the
page of a book, but almost parallel to the page, the page acts like an
efficient mirror and allows you to see the light reflected off the page.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Click here to return to the Physics Archives
Update: June 2012