Light from stars emanates out from it in perfectly straight lines. But if these rays pass close to a massive object such as another star they are bent by its gravitational field. This phenomenon was an integral part of Einstein’s theory of general relativity, and was first witnessed in 1919 on Ascension Island during a total solar eclipse. It was found that light from stars positioned slightly behind the sun could be seen by the observers when once the moon had blocked out the glare from the sun. This proved that the light from these stars must have been bent by the sun’s gravitational field towards Earth.
This object can concentrate this light toward the observer much like a magnifying glass, making the star appear brighter. This effect is known as gravitational lensing. Gravitational microlensing is when a dim object passes in front of a star and its gravitational field focuses the light from the star behind making the distant star appear brighter. The effect of this magnification can be plotted over time to give the magnitude and period of this microlensing;
As the dim object passes in front of the background star its apparent magnitude increases to the point where it is directly aligned between the background star and the observer, this effect tails off as the object continues its transition past this point.
However it is not just star’s that have the gravity required to create this effect on the light of other star’s. Planets too have sufficient gravity to express microlensing effects on background stars, however these are usually observed as a small peak in apparent magnitude close to the microlensing effect of the star it orbits. The peak is smaller as the gravity of the planet is to small to focus the light as powerfully as the star, much the same way as the a thinner optical lens refracts light to a lesser extent than a thicker lens. The peak it also narrower as the period by which a planet is in a position to focus the light from the background star towards the observer is much very short, as the planets is constantly orbiting the lensing star.