A Newtonian pre-introduction to gravitational lenses

Abstract

Understanding the deflection of light by a massive deflector, as well as the associated gravitational lens phenomena, require the use of the theory of General Relativity. I consider here a classical approach, based on Newton's equation of motion for massive particles. These particles are emitted by a distant source and deflected by the gravitational field of a (opaque) star or of a (transparent) galaxy. The dependence of the deviation angle $D$ on the impact parameter $b$, and the geometry of the (source, deflector, earth) triplet, imply that different particle trajectories may reach an earth based observer. Since $D(b)$ does not depend on the mass of the particles, it is tempting to set the particles' velocity equal to the speed of light to get a (Newtonian) flavor of gravitational lenses phenomena. Orders of magnitude are obtained through a non technical approach and can be compared to the General Relativity results.