Gravitational Lensing | Types of Gravitational Lensing | Gravity

Though weakest among the fundamental forces, gravity is the force that makes huge impact on macroscopic level such as formation, shape and movement of heavenly bodies. Also you may know that Gravity can even bend light. In this article we shall learn about a amazing Natural Phenomena called Gravitational Lensing that helps Astronomers to detect or discover most distant galaxies, Planets and even the dark matter. This effect was first measured in 1919 during a solar eclipse; the Sun’s gravitational field caused light deflection thus changing the apparent positions of stars.

Gravitational Lensing

According to theory of special relativity, Space and time are woven together as a single fabric called Space-time. This fabric is deformed or curved by massive objects (heavy mass objects) such as sun; when these bodies are placed on this fabric it causes curvature of space-time.

When light emitted by far away objects such as very distant galaxies pass near these massive objects the light bends due to the curvature made by the objects. When the light comes near the center of the massive object where curvature is larger it bend stronger (Maximum deflection); while when it travels away from the center it deflects minimum. For example; Imagine this scenario, you are the observer on earth and studying the light emitted by distant galaxies. Let's say the light emitted by a distant galaxy bends around a massive object (Sun) and reaches you; when you retrace the path of the light you see multiple images of the distant galaxies. The observer may then see multiple distorted images of the light source. 

These images formed may be magnified, distorted or multiplied by this natural phenomena of lensing; however it depends upon the position of the light source with respect to the lensing massive object. It also depends on the shape of the lensing body. Spherical lensing body provides perfect Einstein ring, Einstein cross (4 distinct images) is provided if the lensing body is elongated and multiple arc like images are formed when cluster of galaxy acts as lensing body. Thus in general Gravitational lensing acts like a lens or magnifying glass through which distant objects can be observed easily, through multiple, magnified and distorted images.

Gravitational lensing Types

Three cases of Gravitational lensing are Strong lensing, Weak lensing and Micro lensing. 

Strong Lensing

When the light source are visible easily with distortions (multiple images), then strong lensing has occurred.It happens when the light source, Observer and the massive objects (which acts as gravitational lens) are aligned closely. Eg. Einstein rings. It is the result of such strongly bent light.

Weak Lensing

In weak lensing the images formed have less distortions, are stretched and are very smaller; detecting distortions on individual galaxies is not possible. Detecting and analysing them is done by statistically analyzing an image of the sky.This weak lensing is used to measure masses of a many objects in the universe. Weak gravitational lensing helps astronomers to analyse the pattern and learn about the distribution of dark matter. Thus helps in investigation of the universe expansion. 

Micro lensing

In micro lensing the amount of light that an observer receives changes with time (changes in apparent brightness of the source) and distortion in shape is not observed. Here the mass of the lens is smaller such as a single star thus making it difficult to detect the distortion.