Chinese Magic Mirrors are really clever 

I imagine in manufacturing, the reflected image is simply masked on the mirror surface then etched with a mild acid which would slightly change the relief on the mirrored side. Once etching is completed it's simply cleaned and polished. This would account for the misalignment and non-matching images. This is also similar to the process which constructs the silicon wafers you mentioned for transistors.

It's easy to intuit that imperceptible variations in the surface would cause a pattern in the reflection, but the convex shape making it distance-independent, that's the real genius. Amazing what they could invent thousands of years ago.

In your first example of an indent in the mirror (2:30) you actually created a retroreflector and the outgoing light would stay parallel to all the other rays.

Grand Illusions! That's a shout out on this channel I never expected to see. Those toys and trinkets that Tim shares from his collection are often a wonder to see.

Okay that ending hologram looking thing is so freaking cool. I'd love to see you go into a deep dive into hologram type things.

Great to see Grand Illusions here! I'm only just now realizing there's probably a few videos you could make using Tim's magic toys. :D

Actually there is one more thing missing in this explanation: The part about the optical illusion. How come, that there are large dark areas with light edges of the same brightness as the thinner dark lines? (or: how are there really large dark areas at all) All the reflected light from this large dark area would have to be focussed in the light edge around it, making it much brighter than light edges of thin dark areas. Answer: There are no "large" dark areas. Our brain interprets them as such. The inner "dark" part of the "sign of the zodiac" is actually as bright as the "light" part of the inner circle. Simply pause the video (in the right frame), make a screen shot, paste it in "Paint", cut out a part of the dark and light area each and paste them on a white surface. You will find that they have almost equal brightness. The "edge effect" really creates just pairs of bright and dark lines. Our brain additionally adjusts the brightness of the areas in between.

One method I've seen at least, involves engraving the image in reverse on the back of the mirror, placing the back of the mirror on a hard, flat surface, and then polishing the front of the mirror to its final mirror finish. The pressure of polishing the front causes the metal above the relieved areas to distort slightly. Upon then bringing the back to mirror polish as well, the image becomes imperceptible to the human eye by direct observation from either side, the image only revealed in light reflected onto a surface.

The ones you can by now are not made in the same way as they were traditionally. The "original" way was decently simple as bronze, while being a metal, is soft enough that surface-differences on one side can easily pass through to the other side when stress is applied - aka when polishing. That can intentionally be enhanced further but just casting it with a relief and then polishing will already give you this result. But to produce them now there are easier methods than painstakingly and slowly polishing them. As many have said - etching is a very simple and fast method. You could also imprint them with a stencil.

The optical physics of this is fascinating, but I'm more excited by the realization that this is what the Mirror of Twilight was supposed to be in TLoZ: Twilight Princess.

Aside from acid etching, you can also just stamp the surface then polish it back; if you light it while polishing in stages you can do it by hand without killing the image (I've seen this done with backed foil). Etching would probably be more consistent for mass production though.

At first, I thought they probably transferred the image on the back by making the mirror, and then placing it on some stiff rubber and giving it a really hard whack, slightly bulging out the areas that were raised on the back. After you pointed out that the images' don't always line up, though, I bet the etching trick is probably correct.

4:43 it might be more intuitive to portray the light rays as bright lines on a dark background :) So bunched up lines would actually be lighter. Regardless, wonderful video as always, thank you!

Took me a while til I realized the light wasn’t passing through the disc. You’re pointing the light from the wall to the shiny side. Then the light bounces back towards the wall

Regarding semiconductor manufacturing, there is even more crucial role for this technique in some processes - mask alignment. For each manufacturing step a different pattern needs to be transfered onto the wafer surface so the first mask usually contains alignment targets for all the subsequent masks.

I remember long ago watching a documentary that included how traditional magic mirrors were made in Japan after the technology was imported from China. The workshop would cast the mirror, with the image on the back. To bring out the image on the mirror side, months would be spent scraping the mirror surface. The maker would feel the surface, scraping off finer amounts, eventually polishing and apply a nickel finish. The presenter did mention that false backs could be added to conceal the original image, they didn't say how. My source for this is the 2005 series "What the Ancients Knew" s01e05 "The Japanese".

the lion at the end is the coolest thing shown in the video lol, i hope he makes a sequel for it :)

I was yelling "etching!" at the screen right from the beginning. It's THE go-to technology even today for creating shallow features after all, easy to control by time of exposure, and there's nothing high tech about it that couldn't have been done just as well in more primitive times.

I read once of medieval Japanese using the hidden image to hide a crucifix on their person, during the period where christianity was outlawed. The explanation given was that the thinner sections were carved away by a draw knife stile of polishing, with an iron knife on a bronze mirror, to produce the fringing pattern.

I was just thinking "this looks like Tim's collection" and then you mention it literally is his😂 I love his channel so much, so it’s a joy hearing his name!

Neat that Grand Illusions supplied you with some mirrors. That aside, it's crazy to think how this was found out thousands of years ago.

I’ve never heard of anything like this but it’s so cool! I love that I never know what I’m going to learn about when you post 😅

I am truly astonished. How in the world did they make these 1000 years ago !? This is amazing today for goodness sake. Thanks for a cool video!

I seem to recall there's a description of how to make these in "Dunninger's Complete Encyclopedia of Magic", Steve should definitely get a copy, it would keep him busy for weeks.

my theory is that it's just embossed onto the mirrored side and they just polish it into a convex shape but the polishing medium is quite soft & flexible allowing it to retain the embossing in the mirror surface

You don't spoil the Magic by explaining how it's done. The explanation on how it's done is part of the magic. Science is a type of wonder

I think that the way that they make it is with masked polishing. you polish the whole mirror surface with the base curve preset, then you mask the image and polish just a litle bit more, the reflected image is then the controlled defects in the polished surface

Earned a sub, very nice video with outstanding explanation of the thematic, pls don't stop doing stuff like this, we need more people like you in this world! :)

No wonder, glad you and Tim know each other, it's like finding out two of your distant friends are actually related.

Steve, we talked about this. If it doesn't guggle, whistle or have a 2D representation of it, I don't know what's going on! (lol)

Enjoyed the video, and the exploration of what is behind a phenomenon I'd never seen before. But, to be honest, the teaser for the future video about transparencies looks amazing! Definitely returning for that one.

The transparency video you teased is gonna be awesome, I’ve never seen anything like that

Wow, it's incredible how those people 1000 years ago could predict that Einstein would be born and put his face on a transparent mirror

The traditional method of making these was polishing a piece of metal by hand on top of a stone with the image you wanted to project. Each one would take days or weeks to make. But I doubt that’s how they make these modern versions you used in the video.

During WWII, my dad spent some time in India, and these mirrors were on sale in the markets. He ordered one that reflected "I love you Edna" for my mum, and he picked iot up two or three hours later. They were made of brass, and brass is a work-hardening metal. To produce the image, they simply tapped out the shape with a hammer and a sheep's foot chisel (a small one, of course), which hardened the surface on the image, then they polished it with metal polish like Brasso. That wore down the softer parts, which had not been beaten, more than the work-hardened parts. I imagine that getting the image right with a hammer and chisel was a real skill - but in such places as India and China, there are many people with finely honed skills.

It made me so happy to hear that you got these from the grand illusion channel.

There is an article and a video in RU-vid where a Japanese artisan explains how at least the traditional process works. The image is indeed produced from the casting on the back side. Then the mirror surface is just ground to exactly right thickness so that the small changes in the metals inner structure become visible. Also Wikipedia article "Chinese magic mirror" has a reference to a book that gives a similar description of the process.

Excellent video as always. Definitely looking forward to the video on the caustic patterns in glass!

I appreciate the fact you are humble which requires honesty opposed to most other producers who act like they know it all

Its videos like this one that reminds me how much I love your content as well as your good friend Matt Parker. Found both your channels at the same time, thru number/computerphile many years ago.

I wonder if there is any similarity to how Schmidt Cassegrain corrector plates are made? Apply vacuum to the embossed side, polish the mirror side flat, then release the vacuum. You can imagine that a suitable technique might result in some parts of the mirror being ground down more or less, resulting in a faceted mirror surface.

These play an important role in one book of Ken Liu’s fantasy series The House of Dandelions. It’s fun to see the science behind them.

Really cool! You could try to make one such mirror. With the technique you mentioned of etching it on and then polishing. Would be interesting to see how it compares!

By mentioning it's convex at the start you set it up perfectly for me to feel really clever for figuring out how it worked before you explained it

I believe it is - or was originally - done by having some design on both sides, with one side then being manually sanded and polished, stopping just short of erasing the image completely. The progress could always be checked by holding a light to it, not only just when it's complete.

I just love these discussions. Even knowing how something works, it can still SEEM magical, knowing that people have been making these things for centuries before they knew exactly how they worked.

When you said it’s “maybe etched then polished” I immediately felt confident that this is the way they do it. I bet they have a mask and they use cotton buds or similar to apply a slight acid of some description and immediately neutralise that acid

you are probably on the right track with the differential cooling. If I had to take a guess the thinner parts that cool slower have a harder micro structure due to larger crystals. When polished, this would result in the minuscule raise to surface compared to the material that cooled faster.

It reminds me somehow the method which it is used to see imperfections in lenses or transparent materials in industries.

Tim is such a cool guy. would be cool if you did deep dives on other tools and gizmos that Tim has. every item a mystery to be solved

Always interesting, feels like I haven't seen you in a while, perfect timing!

I'm happy I solved this in my head before you explained it, solving a puzzle to start the day is a beautiful thing.

Some places have souvenir "penny presses" that take a penny and use mechanical pressure to flatten and re-emboss it with a new image. Some of these are two-sided, but older ones left the back of the penny flat. However, even flattened, it's possible to see what that side of the penny originally looked like, and still read the minting date or text that was on that side of the coin. Perhaps there's something similar happening here - An image is placed on the disc with a normal casting/minting process, then pressed into the elliptical-dish press to flatten out the image. If the disk is only pressed part of the way, the image from the original casting would still be present, just much flatter

What happened to the audio at 0:47?

Once upon a time, I made my own wave chamber. They used to be popular in the 80s and 90s. You pour alcohol and food coloring into a glass container. In my case, it was a mayonnaise jar. Fill the remainder with kerosene, and you have a chamber that will slowly form waves as you tilt it from side to side. Commercial ones had a long, thin chamber mounted on a motor that would tilt it left and right, automatically forming the waves. The peculiar nature of my wave chamber I only discovered by accident, when placing it on my desk, near a small desk lamp with a tiny light bulb (think auto turn signal type.) As the light shone down upon the boundary surface (which had a much larger surface area than the commercial product; 5-inch diameter mayo jar) between the alcohol and the kerosene, it refracted in a similar way upon the wall behind it. You could see caustics in a similar way that sunlight shining through a pool causes caustics upon the bottom and sides of the pool. It was truly mesmerizing, as it appeared holographic in nature, as the surface deformed with wave formation while the strong, single-point light source shone through it. I could rotate the jar and see the shapes changing as if a hologram was being rotated. It fascinated me. I can't imagine anyone else has had the same experience, because I have never seen any reference to this type of phenomenon since then. Maybe I'll try searching on RU-vid. I love your curious videos, Steve. Thank you for dislodging this 41-year-old memory. Cheers.

No idea why it makes me so happy when there are collaborations between fave RU-vidrs. It would be great to have Steve and Tim together.

My guess is that after it is cast an image is stamped onto the back of the mirror, creating a pattern of differing density in the metal. Next, the back would be sanded with a flat, rigged abrasive to create a perfectly flat surface. If it is then polished with something with a bit of flex to it (not unlike a foam sanding block), the less dense areas will abrade slightly faster then the more dense areas. Also, the flex in the polisher would create a slightly concave surface, giving the mirror a slight convex surface.

What if you had a concave metal "stamp" of the desired image that you pressed the soft brass into and then polish it so it's almost invisible. Or maybe an image made of a clear lacquer that distorts the light is covered with one that doesn't so you can't see the difference? Try a solvent on it!

I think it's great the way you use other youtubers that I'm subscribed to like Tim, or Dustin from Smarter Everyday. Great video Steve.

nice, ive never heard another channel mention Grand Illusions.... nice that you worked together.

you cast it, you pregrind it, you press it tightly onto a stone with a leather lining, you polish it, done. magic. relying only on tension while casting wouldn't work because all metal acts inhomogenously in its crystalline structure referred to temperature induced tension, so your result would get wobbly

Once again I find your video topic overlapping with the world of ultrahigh power laser driven inertial confinement fusion reactors. That last transmissive version you showed is exactly what we do to the laser beams right before they fall onto the frozen hydrogen target. We don't use a lion image... as you may have guessed, but rather a random pattern of bright and dark areas across the whole area of the beam. This essentially destroys the phase coherence of the beam and DRAMATICALLY increases beam intensity uniformity or "smoothness" by suppressing laser speckle induced by sub-beam or intra-beam beamlet interference as it is focused from a third of a meter in diameter down to a tenth of a millimeter at the target capsule surface. This increase in beam intensity uniformity is incredibly important in suppressing the in-flight formation of Rayleigh-Taylor hydrodynamic instabilities of the 100 million Kelvin plasma as it collapses to a few microns in diameter. We call these optics "distributed phase plates" or DPPs, and without this technique the maximum fusion neutron yield on the machine would probably be something like an order of magnitude lower. Interested to see the next video.

9:00 I love it when new insights and technologies allow for such beautifully intricate and advanced designs to be created onto otherwise rather mundane and simple materials.

Super cool, pls make a video about those transparent pieces you've shown at the end. 🙏 It's amazing to see all the incredible things already exists, and everything you show in your channel.

As Arthur Clarke said, any sufficiently advanced technology is indistinguishable from magic. Loved the video, Steve!

This is the kind of stuff I like watching on RU-vid, videos of things I had no interest in previously turning into my obsession for the next month.

FINALLY!!! I've been waiting for years for someone to pick up controlled optical caustics. Super excited!

I'm astonished how you pull up random things from all parts of the world and manage to baffle me more each video. This channel is just pure (technical) magic to me. I absolutely lost it in the end with that clear card. Seriously, I was screaming "WHAT?!?!?!?!" at my screen - a kind of physical reaction that I rarely have when watching any video! Thanks for bringing that kind of magic into my life, Steve! I'm sitting here, being just baffled as to how somebody can scratch all the itches in my brain so perfectly....

Interesting, reminds me of the lithophanes you can make with 3D printers.

This video was amazing, especially because you explained it extremely well!

As a machinist, I have seen many examples of how cold-formed metals have a lot of stress residing in them, especially if they were never annealed or stress-relieved after forming. For example, a smallish rectangular bar of common "cold-rolled" steel such as type 1018 will have stress that causes it to visibly warp outward like a slight banana peel effect if you split it down its centerline for 3/4ths of its length on a bandsaw. It has always been my observation that this stress is higher near the surface of such cold-formed bars. So, say you had a bar that measured 1/4" thick and 3/4" wide, you could greatly reduce the amount of spreading you got at the bandsaw cut, if you removed just 1/16" from each edge, making it into a 1/4" by 5/8" bar before splitting it. I suppose that if the mirror blank was "coined" by stamping under great pressure, with a considerably raised pattern on what will become the mirror surface, then there could be a planned pattern of distortion when that raised pattern was machined off later. Depending on how you machined it off. Of course, if this type of mirror dates back thousands of years, then cold forming seems unlikely on something so large. But similar stresses could be set up in the metal by having considerable variations in thickness created by traditional sand-casting techniques, and then at some point quenching the metal from a high temperature. So that is my guess: They did not rely on the raised pattern on the back of the mirror, but rather a raised pattern on the FRONT, which was then machined off.

Great content as per usual mate. Really enjoy the explanations that I can understand. Also nice flex with your mirror shot 😉

Very cool, am interested in the transparent glass projection technique as well.

This channel never ceases to amaze me in the new fascinating topics in videos. Always top notch quality.

The instructions I've seen involved work hardening the bronze by use of a punch, then polishing flat. There would be tiny raised areas where the hardening has occurred.

Absent powerful calculations and clear understand of photon behavior, it is mind-bending to consider the effort, trial, error, and precision required to create the techniques required for these magic mirrors.

Your videos usually blow my mind. This was amazing.

One of my proudest subscriptions. Thank you, Steve!

a crossover with tim from grand illusions is the absolute last thing i was expecting, tim is a treasure

You ended it on a cliffhanger??? That lion looked like it was a hologram. That's so cool.

I assume it was cast, stress relieved, and then cooled (or heated) for polishing. I would assume the polishing is done chilled. When it returns to room temperature, the thickness differences cause very minor distortions. Perhaps it needs to be an original image on the polished surface side vs the distortion due to the thickness on the back side. The thermal change before polishing should still translate after the thermal delta to room temp is "released" But I think management of casting stresses would be quite important

I'm still waiting for the video on the Rayform, and how to obtain one. That's really awesome.

Wow! What a neat thing! And every time I see a neat thing, I wish I knew how it worked. In your videos I get to find out immediately after being introduced to the neat thing! Thank you

This reminds me of the Cross Atlantic Communications Cable problem The length of the cross Atlantic cable meant that, for traditional telegraphy, you needed very high voltage for your comms link, which burnt through the cable's insulation. This was solved by using a Mirror Galvanometer. The very small changes in the rotating mirror meant that much lower voltages could be used.

I love this breakdown of very old toys and niknaks that you've been doing... So cool and fascinating!

when polishing and lapping diamonds for record cutting styli we found that the temperature at which we grind would affect the surface flatness ( fractions of a micron ) when its service temperature differed from its cutting temperature. In this way we could grind concave or convex surfaces on an essentially flat lapping skaife. As the surfaces we are grinding are less than 0.1 mm^2 laser reflection would reflect individual crystal lattice dislocations

The first thing I thought of was uneven tension in the metal, I know when people talk about casting (and glass making) that sharp edges and uneven cooling create high and low tension areas in the material. I suspect thats why the castings are so deep, you could pour water or oil into the shallow parts to increase the temperature difference.

Ahhh tip top Tim! Love when people give his channel kudos

From the THUMBNAIL I was thinking of Tim. I started the video, and then a minute later you can imagine my delight.

I'm very satisfied that the real object deals with the interaction of a sheet of light hitting a plane, and the animations deal with a line of light hitting an edge, continuing your trend of making a 2D version of a 3D thing to more easily explain it

You should do a video on moire fringe patterns. Its pretty intuitive if you look closely but initially it's just a really neat looking patterning (especially with a moving subject). I did it with a body buckling under load. Its also something that is likely going out of vogue in favor of something like DIC (which is also really neat)

8:56 this is one of the coolest things i’ve ever seen, especially the rotation

A similar effect could be achieved by impressing the design to be reflected with a forging process, then grind flush and polish. The harder metal would be more resistant to being polished away, creating a raised area with a decently smooth transition between the higher and lower areas.

Oh wow that upcoming video looks so good! I've always thought about manipulating caustics like that

Cool mirrors and excellent video! 👍 Thanks for sharing!

Well... Didn't expect to learn such thing today :D Thanks Steve!

I think when the backside is pressed the front-sides surface press has the other image. I assume the backside is hotter and softer (different material) and thus is pressed deeper. Another way would be using a weak acid for the mirrorside and some anti acid gel for the raised parts. Acid could work slow enough to make the dent not visible. Maybe it's very weak acid removed after 1 minute so only a few layers of atoms are removed

I'm loving the grand illusions crossovers, would be really cool to see a video with Tim in too

i am a geologist specialising in trace fossils but also have a solid backgrownd in minerals and how they form the theory is on point but it will take alot of calibration to work out the right timing and thickness to produce variable crystal sizes that result in the correct level of deflection. Very impressive this is the first thing that really impressesme from a te4chnological point of veiw this is the same as the mechanism from the mediterian. it suggests that there is a lost knowledge or something like if you used this to make call signs, and you have a amplifier then you could send this 160 km on a clear day.

I love your videos so much. Real deep science and observation. Have you ever thought about a video about why certain plastics have a crinkle sound? Like cellophane, of chip (crisps) packages? Or is it obvious and I’ve missed it…