This scenario is based on the deviation of the light by massive objects. The light is emitted by a background source (star or galaxy) and received on Earth. In the absence of deviation, only one ray is received on Earth, along the direction to the source. If a mass (the lens) is located between Earth and the source, general relativity states that light rays are deviated towards Earth. If the source, the lens and the observer are perfectly aligned, a luminous circle will be seen.
If there is a slight misalignment, this will result in multiple images, and an enhanced luminosity according to the mass of the lens. The students will discuss the geometrical aspect of this phenomenon, the different orders of magnitude of the lensing, and then apply this to a case of ‘microlensing’ to infer the mass of a lens in our Galaxy.
The deviation of light
It is well known that light-rays travel along a straight lines. But why is it the case?
When a star shines (or when the lamp shines), it emits particles called photons in every directions around it. The light that one receives corresponds to the beam of photons whose track ends up in the telescope or in our eyes. So this is only a small portion of all photons emitted by the source.
Those photons always choose the shortest path to go from one point to another (this is called the ‘geodesic’) in order to minimize the loss of energy. In general, the space that surrounds us is well described by a flat grid (this is the ‘Euclidian geometry’), where geodesics are straight lines. If we were in a special position, where the particles were obliged to stay on the surface of a sphere, then geodesics would be curved.
Objectives
The learning activity proposed is significant because it favors creatively the pupils’ knowledge concerning the concepts of: movement, gravity and light (emission and reception) in an unusual context of general relativity.
It also challenges common conception of space. It also offers to the pupils the opportunity to test their own scientific hypothesis and also to test these hypotheses’ applicability in new learning contexts.
Download the PDF complete version by clicking on the following link: educational_content_microlensing.pdf
This exercise uses SalsaJ application.
Educational content developed by Institut d’Astrophysique de Paris
Validated by Discover the Cosmos + Microlensing
Last update | 20/06/2013