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<-   Gravitational lens   ->
images/lentille.gif
A very distant star moves exactly behind another star around which a planet orbits. The nearby star focuses the light from the distant star, which increases the luminosity. The planet focuses the light from the distant star too, which creates two luminosity peaks visible before the maximum.
Copyright : Paris observatory / UFE

This is an application of Einstein's theory of gravitation. This theory predicts that when a body of mass M (the deflector) is at a distance of b projected on the sky (the impact parameter) within the line of sight of a distant source, the apparent brightness of this source is amplified by the factor A=racine(M/b). When the source and the deflector are displaced from each other on the plane of the sky, the impact parameter b varies with time. Therefore this is the same for the amplification factor, which becomes a function of time A(t).