Massive objects deflect light (Einstein 1915, confirmed 1919). Einstein radius θ_E = √(4GM/c² · D_LS/(D_L·D_S)). At perfect alignment: Einstein ring. Microlensing amplification A(u) = (u²+2)/(u√(u²+4)) — a symmetric light curve peak. Bullet Cluster 2006: gravitational lensing maps proved dark matter exists separately from hot gas. Gravitational lensing measures H₀ independent of the CMB, helping resolve the Hubble tension.
🚀 Simulate in the browser