# FLP: Derivation of reflection coefficient

In Vol. I Ch.33 of Feynman's Lectures (http://www.feynmanlectures.caltech.edu/I_33.html), 33-6, the reflection coefficient as a function of angle was derived.

I am confused about the part where it said the component of A perpendicular to B (Acos (i+r)) has the right polarisation to produce B. Geometrically I obtained Asin(i+r) instead but I am unsure about why the physics is so.

Similarly, it was also mentioned that the component of A normal to the dashed line (Acos(i-r)) is effective in producing the field of magnitude -1. I am confused as to why this component produces the field of magnitude -1.

Thanks.