The fact that light is bent inward by gravity is called “lensing” because the clear lenses found in many reading glasses have much the same effect, even though the cause is quite different. A clear plastic or glass lens can bend light inward through the effect known as refraction, in which light, entering a transparent material, slows down, and consequently changes direction. (Many websites give an explanation as to why slowing of waves, whether those of light, sound, water, or anything else, can cause them to change direction; for instance, see here.) A circular lens that is convex (i.e., shaped as in the lens of the eye, with thin edges and a thick middle) bends the directions of light inward. This can have several effects, including:
- an object placed at just the right location, the location of focus or “focal point”, can appear to be magnified and in focus.
- a glowing object seen in the glass may appear much brighter than it is; it is on this principle that the Sun’s light, if focused onto a piece of paper, may cause it to burn.
- a small object placed on the line between the viewer and the lens’ center, but beyond that location of focus, can appear as a ring.
An object with a large gravitational mass compared to its size can have a substantial effect on the shape of space (really space-time) and this can cause light to bend in much the same way as the refraction of light by glass. We refer to it as a gravitational lens.
As an example, if a compact source of light, perhaps a distant galaxy, lies exactly behind a symmetrical gravitational lens (perhaps a less distant galaxy or cluster of galaxies), the light from the distant source can appear to be a ring. While a perfect ring is unlikely, since the lensing object is rarely perfectly symmetrical and the lensed object is rarely precisely behind it, nearly complete “Einstein rings” have been observed numerous times by astronomers.
But there are many other examples of gravitational lensing observed by astronomers, including some less dramatic and beautiful but far more practically useful. Lensing is used to search for planets, black holes, clumps of dark matter, and even the overall distribution of dark matter. Thus gravitational lensing has many applications in astronomy, as covered on many telescope websites.
