What is gravitational lens or gravitational lensing?

Gravity is nothing but, a curve in the space-time. 

So try to imagine that what happens when light travels trough space around a massive object lets say a galaxy.

What would happen to light and why?

Ok, if you get the point then it's good but, if you don't then let's try an example.

Let's consider two white balls, one bigger and heavier and the other is smaller and lighter are placed on two different surfaces with different conditions.

In the first picture below the big white ball is placed in the middle of a hard-top table and the small ball is rolled by the side of the big ball. Both the balls remains unaffected by each other.

In the second picture the big ball is placed on the middle of a flexible material, so the area around the ball bends due to the ball's weight. When the small ball is rolled into the surface of the material then the ball starts to follow the curve rather than maintaining a straight line.

In the third picture the small ball is rolled out with high speed then it would escape the curved surface created by the big ball. Now let the small ball be a photon of light so it will also show the same effect if there is enough mass present.

So by bending of light by distribution of matter( like a galaxy or clusters of galaxies and dark matter ), we can see objects that we couldn't see normally if the bending effect was not present. This is called 'gravitational lensing'. The matter which causes light to bend is called 'gravitational lens'.

Let's say there is massive galaxy present between us earth and a quasar. The galaxy is so huge and it contains billions of stars and clouds of gas and dust that it's impossible to look through it. It's like trying to look through a stone wall. But as we can see in the picture below, due to gravitational lensing, we can actually have multiple views of the same object.

Source - gsfc.nasa.gov

Unlike optical lenses, gravitational lenses have no focal points but, focal lines. If the source of light, the lensing object and the observer lie in a straight line then the light source will appear as a ring around the lensing object. It is called as an 'Einstein ring' as in the image below. If there is any misalignment then it will appear as an arc.

Test it for yourself.

Well if you look through the bottom of a wine glass or any other glass with a thick bottom you can see the, lensing effect or the ring we just talked about for yourself. Just take a plain white paper and make a mid sized black dot on the middle of the paper. Then take any type of glass mentioned above and look from the top of the glass through the bottom at some distance from the paper. As you will move the glass over the dot you will see the lensing effect and the ring gradually.
  

Source - gsfc.nasa.gov

Source - astro.berkeley.edu

This is the image below taken by Hubble Space Telescope  We can clearly see the lensing effect and the arcs present in the view.

For more info, please visit - arxiv.org