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Name: Roxana
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: 5/31/2005
Question:
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?

Replies:
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


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