Gravitational Lensing

-by Saieesh, Core Member, SEDS Celestia

Ever wonder how we could see behind the Sun? Ever wonder how we could see so far beyond in the universe?

It is because light bends!

First predicted by Albert Einstein, illustrated in his General Theory of Relativity, Gravitational Lensing or GL, is a property of the heavenly bodies through which we can observe astronomical bodies and study them through the eyes of gravity.

But, how does GL work?

Initially, it was thought that light can only travel in straight lines and is unaffected by gravity. But then to prove Einstein’s General Theory of Relativity, an experiment was conducted by Arthur Eddington during the total solar eclipse of 1919, to prove that light-bends. The experiment required special conditions and special telescopes to detect the cluster of stars that were behind the Sun (from our perspective). During the eclipse, the results obtained were matched with Einstein’s predictions and, thus the General Theory of Relativity was proven, and the light from the stars bent due to the gravity of the Sun.

Gravitational Lensing in Action Credit: NASA, ESA & L. Calcada

Furthermore, there are two types of GL: strong lensing and weak lensing.

In strong lensing, the image of the celestial body is magnified. This magnification is obtained when the observer is located at a correct distance, and the rays from the body converge to form a magnified image. Through strong lensing, gravity acts like a telescope.

In weak lensing, the light rays are not deflected enough to be magnified and are thus distorted. With the help of weak lensing, we can map out the locations of all the clusters of nebulae, galaxies and other celestial bodies in the Universe.

GL also helps us to discover new planets through a method called “Microlensing”. When two stars “cross each other” (from our point of view), this effect is called Microlensing. If the closer star is host to exoplanets, then the predictions of Microlensing are a bit distorted. And, just under the right conditions, astrophysicists can ‘see’ the exoplanet.

Credits: Grant Christie, ResearchGate

Due to the discovery of GL, the study of dark matter has also become possible. Dark Matter has a gravitational field. When the light, deflected from the celestial body is different from its predicted, we realized that something else was in play. There was something that was causing this variation. And, thus, since we could not see this something, it was called “Dark Matter”.

Due to Gravitational Lensing, we get beautiful pictures of Black Holes; such as this one:

Credits: Astronomy Stack Exchange

From GL, we can predict the speed at which galaxies grow, the densities of the celestial bodies and discover new celestial bodies.

Gravitational Lensing has been a wonderful discovery like wearing a pair of spectacles helps us see better, similarly, GL has helped us gain insight of the workings of the universe a little bit better.

If you wish to explore more into Gravitational Lensing (the theoretical part), then check out International Centre for Theoretical Sciences’ lectures on the same.


  1. Gravitational Lensing | Center for Astrophysics
  2. ESA Science & Technology – What is gravitational lensing?
  3. Gravitational lensing – Curious.
  4. Previous Gravitational Lensing
  5. What’s that ring that only now appears in black hole illustrations?
  6. Gravitational Lensing
  7. Gravitational Lensing in Action – Spitzer Space Telescope

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