# How to explain gravitational lensing to a layman?

This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness.

I am accustomed to explaining optical refraction in terms of wavefronts and the fact that light moves at different speeds in different media. For example, I explain an optical lens by saying

A plane wave that is incident normal to the flat surface of a plano-convex lens will propagate slower inside the lens. This means that the portions of the wavefront that exit the lens first, near the edges, will end up ahead of the portions that exit near the middle of the lens. This results in the wavefronts downstream from the lens having a concave curvature. Because the wave propagates perpendicular to the wavefront, it converges to a point.

I would like to use an analogous explanation to describe the effects of a gravitational lens. Although the local speed of light is invariant in a vacuum, gravity causes the local frequency and wavelength of light in an initially flat, monochromatic, wavefront to vary with radial distance from a gravitating mass. When the light wave has passed far enough downstream from the gravitating mass, the frequency and wavelength return to their original values, but the wavefront is curved because of the changes experienced by different parts of the wavefront while passing through the gravitational field of the mass: the parts of the wavefront near the middle are delayed relative to those near the edges.

I've been trying to refine this explanation, but am running into a complication that, thus far, is beyond my skills. I understand time dilation in terms of gravitational redshift and blueshift. The complication is that both the wavelength and frequency of light (from the perspective of a distant observer) would seem to be affected by the gravitational field. It's not obvious to me how wavelength is affected by gravity. Locally, the wavelength change should balance the frequency change in such a way that the speed of light is $$c$$. This suggests that the apparent wavelength as judged by a distant observer should also change.

I don't know how to explain this in an intuitively satisfying way, probably because I don't understand well enough how a distant observer can remotely measure the length of something that is deep in a gravitational well. Any help will be greatly appreciated.

• Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency. Jan 2, 2019 at 3:26
• The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens. Jan 2, 2019 at 3:28
• @Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays. Jan 2, 2019 at 3:38
• How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
Jan 2, 2019 at 3:41
• I think the 'slower in different media' thing is a helpful at the naïve level. Although the speed of light is constant for local observers those observers have clocks which 'run slowly' as seen by far-away people if they are deep in a gravitational well. So, in fact, light 'moves more slowly' deep in a gravitational well as observed by people far from that well (in flat space). And that's what a lens does.
– user107153
Jan 2, 2019 at 10:55

I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go.

When I explain about gravitational lensing, I use an approach like this:

1. The equivalence principle predicts that light should also fall in a gravitational field. (And experiments on Earth confirm this, but the details are tricky.) So if a distant star/galaxy and a nearby star/galaxy are along the same line of sight, light from the distant object that would have missed us can get bent back towards us by the nearby star's gravity, similar the way that a magnifying lens makes parallel light rays converge. I usually end up drawing some variant of the below:

[source]

Beware that most people's experience with magnifying lenses is to look through the lens at a virtual image. Looking through a magnifier with enough distance to see the inverted real image is a surprise for many adults, and putting your eye at the focus is just plain strange-looking. So you may have some intuition failures to cope with if you use this approach.

2. If they get that gravity can bend light and make images of things appear in the "wrong" place, I ask them to remember a time they walked through a parking lot and noticed a dent on a shiny car. You don't actually see the dent. What you see is the reflection in the car of the car's surroundings. The reflection in the body of the car is distorted a little, because a car isn't flat like a mirror. And where there is a dent, there is more and different distortion. Sometimes you'll see the same object reflected more than once, from different sides of the dent. Most people have seen this enough times that they have a pretty good idea of the size and shape of the dent just from this reflected-image information: walking up to the car and touching the dent is not usually a surprise. And in the same way, an image like this one

[source]

lets you say things about the distribution of matter in the foreground galaxy cluster that's doing the lensing.

This little spiel has served me well.

• As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob
Jan 2, 2019 at 3:47
• Not a bad idea. I'll try to formulate that question tomorrow. Jan 2, 2019 at 3:51
• I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away-- Jan 2, 2019 at 4:59
• I said that incorrectly. In those papers, the effective refractive index is modified by an amount (delta n) proportional to the gravitational potential. Jan 2, 2019 at 11:57
• You don't need to invoke the words "equivalence principle". Just say gravity attracts light, or gravity bends light. Nov 5, 2020 at 9:51

Although these papers (1), (2), (3), (4) explain mostly from a ray perspective, they do derive an effective refractive index that depends on gravitational potential. It is easy to explain gravitational lensing from either the wave perspective or the ray perspective, based on the effective refractive index $$n$$: $$n=1− 2Φ/c^2,$$ where $$Φ$$ is the Newtonian gravitational potential. This is a first-order approximation; some of the papers provide a more exact formula.

"This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness.

I am accustomed to explaining optical refraction in terms of wavefronts and the fact that light moves at different speeds in different media. For example, I explain an optical lens by saying..."

"I would like to use an analogous explanation to describe the effects of a gravitational lens."

I have found a very simple and efficient way for you to teach your students a close relationship between gravitational lenses and classical refraction, which perfectly suits this request. I think you (reader) will also appreciate this experience perhaps unprecedented.

Did you know that bubbles floating on the surface of the water produce a lens capable of converging the light rays to a point at the bottom of the bowl regardless of its depth? Up to two meters, at least. The light that converges to a point at the bottom of the bowl does not pass through the bubble, but externally, in that elevated region (curve) due to surface tension and capillarity. If we change the bubble by an opaque cylindrical object, we get the same result as this Picture shows.

See that the light from the ceiling lamp of the room, passes around the cylinder and then creates the image in the bottom of the bowl. There is a second image produced by the lens that surrounds the bubble that floats on the surface of the water.

This mechanism is equivalent to that which occurs in gravitational lenses and allows us to observe the "Einstein Rings".

If you put a mirror in the bottom of the bowl over the image, you will only be able to observe an "Einstein Ring" when the image in your eye is in the cylindrical-source direction. This image is amazing. We attenuate the light from the ceiling and with this, we get this sequence of images that mimics the "Einsten rings" with great perfection.

Einstein's Ring

See now another photo of this experiment where an obstacle has been placed between the light source and the cylinder. This obstacle also does not impede the passage of light, and its shadow shows its alignment.

In the photo below another obstacle was placed between the image and the cylinder. The image remains integrated.

A gravitational lens does just that: it can make a signal transpose an obstacle in the same way as the refraction of light in water. I think not just with the light, I believe the same also happens with gravitational waves. Perhaps a massive body situated between the emitter focus of the wave and the LIGO (Laser Interferometer Gravitational-Wave Observatory), can maximize it's effect. In this case the "obstacle" can function as an amplifier.

Now, I want to show the optical aspects involved in this experiment. In this figure the effects are maximized and angles and measures are out of proportion and serve only to get a sense of how I am understanding this phenomenon.

Bubbles on the surface of the water provoking luminous images as shown in the photos, is very common in household utensils, sanitary vessels, swimming pools and in large quantities in the beaches. I know this phenomenon for a long time, but it was at the end of this year 2018, that I made this experience and I liked it a lot because it is revealing.

In this video:

Gravitational Lens

The spiral of a galaxy deforms itself visibly as it passes behind a black hole. This effect can also be simulated in our experience, and very faithfully. To do this, simply place the mirror in place of the image and change the light source for a long rod of approximately $$30 cm$$ (length) and $$4mm$$ (diameter). The rod can be placed one meter high, and should slide horizontally, rotating between left and right, so that it passes vertically from the image with the cylinder. In doing this, the image that is obtained is identical in its details with the suggested video. I got good images that reveal the great similarity between the images of the suggested video and the experience.

If you know more about this phenomenon that occurs at the base of the bubbles that float on the surface of the water, and you can inform me, I thank you.

• The effect you're seeing is due to the meniscus that forms in the liquid surface near the edge of the bubble. This paper [researchgate.net/publication/… has some beautiful illustrations of the surface shape around a bubble. Jan 6, 2019 at 2:47
• @S.McGrew - Thanks for your information. Very beautiful illustrations. Today I put more interesting photos about the experience. Jan 7, 2019 at 22:27
• @S.McGrew - There are one important difference (about chromaticity) betwen gravitacional lens and single refration. Here [link] (physics.stackexchange.com/questions/7250/…) you can see more details on TED's answer. Jan 9, 2019 at 0:31
• Understood: In the non-vacuum version of Maxwell's equatuons the permittivity and permeabilty constants are not simply functions of position; they are also functions of frequency; and frequency dependence will be absent in the "effective constants" induced by an interaction between gravitational and electromagnetic fields. Jan 9, 2019 at 0:53