The image of point P(x1, y1) in line ax + by + c = 0 is Q(h, k) where $\frac{h - x1}{a}$ = $\frac{k - y1}{b}$ = -2$\frac{ax1 + by1 + c}{(a^2 + b^2)}$. The foot of perpendicular uses the same formula but with factor -1 instead of -2. Geometrically, the foot F is the midpoint of P and its image Q. Common applications include finding the image of a point in a line, the equation of a reflected ray (using the image of the source in the mirror line), and the image of a line in another line (by reflecting two points and joining their images). For reflection in coordinate axes: image in x-axis changes (x, y) to (x, -y); image in y-axis changes (x, y) to (-x, y); image in line y = x changes (x, y) to (y, x); image in origin changes (x, y) to (-x, -y). Light reflection problems are solved by finding the image of the light source in the mirror line.