Einstein ring, dual images & light curve as source crosses θ_E
The Einstein radius θ_E = √(4GM/c² · D_ls/(D_l·D_s)) defines the angular scale of microlensing. A point source crossing the Einstein ring creates two images whose combined magnification A(u) = (u²+2)/(u√(u²+4)) diverges as u→0. Peak magnification for minimum impact parameter u_min: A_max ≈ (u_min²+2)/(u_min√(u_min²+4)).