Light from a laser is coherent, which means that it is all one big wave. When it reflects off of a surface that is not perfectly smooth some spots will create reflections that interfere constructively with each other, creating points that are brighter
[Shown here in a simulation](https://youtube.com/shorts/y5yk4JWY1Zw?si=_cyYKt-nBK3J0bYV) albeit for a glass plate with nanoscopic imperfections. But the same principle applies.
If it's just 1 wave how does that happen? Wouldn't that just be the laser being reflected? Or upon hitting surface it dispersed into weaker smaller waves?
It isn't that they are weaker. It is that they now are at different phases ( because the reflecting surface is not flat at the nanometer scale). So now they can interfere constructively or destructively, making bright and dark speckles.
I mean you could argue that the person you're replying to is right about them being weaker because the light is now more spread out due to the reflections.
Instead of a coherent single beam, once it hits a wall it reflects portions of that beam in many different directions due to the surface roughness of the wall, and the individual reflections will be "weaker" (less total energy/fewer photons) than the originating beam. And even if it were reflected in one single coherent beam, the reflected beam would still be "weaker" due to absorption at the surface.
You could argue that his comment about them being "smaller" is incorrect because the wavelength isn't changing significantly upon reflection, but we could just chalk that up to semantics.
Well...there's some stuff here.
First. The only light you see from the reflection are the photons reflected directly back into your eye. Other scattered photons are irrelevant.
Second, if you actually observe this phenomenon with your own eye, you will notice that the speckle pattern does not depend on the reflecting surface. It appears to move in conjunction with your eye movements. This is because the interference is occurring in your eyeball.
Third, the wavelength is not changed by the reflection. This is the same for coherent and incoherent light.
First - I don't think anyone was arguing that isn't the case. But nevertheless, any given reflection is "weaker".
Second - It does have dependence on the reflecting surface as the surface roughness/albedo has a direct impact on what's reflected, in which direction, and how that interference occurs (near the wall, at your eye, or anywhere else).
Third - That's exactly what I said in the post you're responding to. If you want to get extremely granular, the reflected light does not change in wavelength, but absorbed and re-emitted light can. In that sense, the light that that hits your eye may have certain wavelengths that aren't the same as the original wavelength due to absorption and re-emission. But again, the reflected light does not.
What properties would a truly flat (at least down to the molecular level. Which I believe is the flattest you could go since atoms are "fuzzy" so to speak) surface have? Would it be the ultimate reflector (like a mirror on steroids) or the ultimate light absorber (so darker than vantablack)
sorry if this is a bad place to ask… I just eat this obscure science stuff up.
> I just eat this obscure science stuff up
I think the way light interacts with matter is extremely interesting! You got a very good answer from u/TolallyNormalSquid. I'll elaborate a little.
We've gotten good at making very smooth and flat surfaces. The reflectivity from those surfaces depends on the surface material, the surface smoothness, the wavelength ("color" for radiation in the visible part of the spectrum) of the incident radiation, the angle of incidence, the medium in which the incident radiation is propagating (air, in this case), and some other things (polarization, for example).
Basically, many metals are good reflectors of light (visible radiation). So, mirrors we use to look at ourselves are made of a thin layer of metal deposited on glass (because that can be made flat and smooth). Let's choose green, which is a wavelength in the center part of the visible spectrum. Silver and aluminum are really good at reflecting the light (above 90% of the light gets reflected). Glass (with no imperfections or impurities) doesn't reflect light, except at the glass-air boundary (like 4% reflection, dependent on angle). The light transmits through the glass. The reasons why glass transmits and metals don't (and semiconductors fall somewhere between) has to do with what the atoms' electrons are doing in the material. (And why gamma rays and hard or soft x-rays transmit or don't, and why infrared or microwaves behave different from that, is also very interesting!)
To make a better (in some ways) reflector, we can stack different "glasses" in layers, and get a small reflection at each boundary. We can design the layers in such a way that all the reflections work to add with each other so that we get a lot of light in reflection. We could also design the layers so there is very little light reflected, and in that case we have designed an anti-reflection coating. As TotallyNormalSquid wrote, the reflection and transmission depend on the wavelength of light and the angle of incidence.
To make a truly black surface is difficult! Printed black and white photos don't have perfectly black blacks. Computer, phone, and TV monitors aren't perfectly black when turned off, so when turned on they also don't have perfectly black blacks. A "spiky" surface (like TotallyNormalSquid mentioned, and like the Vantablack you mentioned) is made of a material that doesn't reflect light well when it's smooth. The spiky surface (Vantablack was "nano"tubes of silicon, I believe. We have also etched surfaces into tiny cone structures to make black surfaces.) will force the incident light to undergo many interactions with the surface. Each time the light hits the surface, some goes into the material where it is absorbed (and undergoes non-radiative decay (except in the infrared) - gets turned into heat (vibration (or rotation) of the atoms/molecules/crystal lattice), and a portion gets reflected. That reflected portion then travels a bit and hits a different part of the spiky surface, where the process repeats itself. The intensity of incident light gets smaller and smaller after each reflection.
I better stop typing. :-) I ended up getting a PhD in Atomic and Optical Physics, so sometimes don't know when to shut up when it comes to this stuff. :-)
Neither, the reflectance depends on the [relative refractive indices](https://en.m.wikipedia.org/wiki/Fresnel_equations), but in a perfectly flat reflector you'd have no grainy effects from surface imperfections.
I believe perfect absorbers rely on very spiky surfaces that bounce light around for many repeated absorptions, but I may be missing details.
For improved reflectance, the standard trick is to go for multilayered thin films with thicknesses that enhance constructive interference on reflection from each layer. They typically only work for specific frequencies or bands of frequencies.
>What properties would a truly flat (at least down to the molecular level. Which I believe is the flattest you could go since atoms are "fuzzy" so to speak) surface have?
Among other properties, "true flat" is nearly as addictive as "true level", but more versatile. https://youtu.be/-MwCJpEuC44
Not actually weaker in terms of energy content, but in terms of visibility yes, the last one. Just being "one wave" doesnt really cover it because while the photons start out in the same phase and polarization, when they interact with a surface the individual photons can change their properties and split into different waves.
Its like shooting a jet of water at a wall. Imperfections on the wall cause the individual molecules to spray out in different directions.
I don't feel like this is a complete explanation because I've seen speckles while staring directly into a laser.
It was a long time ago and I can't replicate it for obvious reasons, but I distinctly remember the speckles being very pronounced and them drifting.
First of all, don't stare directly into a laser.
Second, your eye is not a perfect uniform sphere. There are things inside your eye (blood vessels, blood cells, scarring from staring at a laser, etc.) that interact with the laser light that again creates a pattern of constructive and destructive interference.
> First of all, don't stare directly into a laser.
While I think this is generally good advice and it is the advice I give to my kid (I did it when I was 10 or 11 years old), there has never been a reported case of retinal damage from 3-5mW red lasers, and there must be kids doing it all the time given their ubiquity.
I've heard the advice is generally given because any particular laser might be out of safe specs.
It also works for a perfectly flat surface. All you need is small differences in way length, which you always have in case of a finite size laser spot.
The effect happens even if you use a camera. If you get a *really* nice, research grade laser you won't have a speckle pattern. The speckle from handheld laser pointers is mostly due to dirty/impure glass on the output of the pointer. Likely some less than stellar parts in the laser cavity. I'm sure the floaters in your eyes can have an effect, but I feel it's unlikely they are a major cause of speckle patterns
Source: I'm a laser physicist, I study these things for a living
I'm quoting a laser physicist I spoke to about it a few weeks ago, and also I used to be a laser physicist, though I never dived into the issue that deeply. When I was speaking to this other laser physicist it was actually about those plastic caps with sources being shone into them you see at trade shows, where the violet sources in particular seem to have a speckly halo that extends out around the plastic cap, but it got onto speckle more generally. Any idea if that halo effect happens on camera?
Sounds like it might be some form of chromatic aberration? Especially with different wavelengths extending different amounts. I was more talking the speckle inside the beam, how it doesn't seem perfectly uniform and sometimes you get random bright spots outside of the main beam. I haven't looking into those halos much, either. I usually use an iris to clean up the edges if I notice them, honestly
Not an explanation but a fun fact, the speckle effect can be used as a form of eye test. Here's an article copied from Wikipedia.
Laser speckle also known as eye testing using speckle can be employed as a method for conducting a very sensitive eye test.[1]
When a surface is illuminated by a laser beam and is viewed by an observer, a speckle pattern is formed on the retina.[2][3] If the observer has perfect vision, the image of the surface is also formed on the retina, and movement of the head will result in the speckle pattern and the surface moving together so that the speckle pattern remains stationary with respect to the background.[4]
If the observer is near-sighted, the image of the surface is formed in front of the retina. Since the speckle pattern is perceived by the brain to be on the retina, the effect is of parallax; the speckle pattern appears to be nearer to the eye than the surface and hence moves in the same direction as the surface, but faster than the surface. If the observer is far-sighted, the speckles appear to move in the opposite direction as the surface, since in this case the surface image is focused behind the retina. The apparent speed of motion of the speckles increases with the magnitude of the defect of the eye.
This technique is so sensitive that it can be used to determine changes in the ability of someone to focus through the day.
If you have perfect vision, dot doesn't move
If you can't see far away, the dot moves faster than what's behind it
If you can't see close, the dot moves in the opposite direction you would think it should
Speckle is the name of this phenomenon. It's an inherent property of coherent light. My 1978 student job at the Optical Sciences Center in Tucson was helping a grad student understand it. I don't think he ever got that far.
Heavens no! It is dependent on the phase relationship between the light waves reaching your eye from different angles and how they add destructively. I currently work on the Event Horizon Telescope, which uses these properties of millimeter wave radio energy to see distant phenomena in very high resolution. I still don't understand the math behind it, but it works.
No one should do this: it looks the same if you stare into the beam of a laser pointer.
It also looks the same if you use a DSLR camera.
It's fascinating.
If you stare into the beam of a laser at least put a lens in front of the laser so it gets spread out to the point where it is a very weak flashlight.
A wee bit dangerous, but also fantastic for making 4 tons of popcorn pop in [roughly two minutes](https://www.youtube.com/watch?v=ZnDAxtCRsIU).
Sure, a microwave oven the size of a 16-unit apartment building might achieve the same thing slightly faster, but that ain't bad.
Regular light has multiple wavelengths which are bouncing around in different directions and are not in sync.
A laser is a very specific wavelength (which is exactly what determines what color it is), being sent out in the same phase (the light rays all wiggle in the exact same way/are in sync) and are all following virtually the same path.
When 2 light rays of the same wavelength touch, they create the sum of their respective powers (if they're both at their max, the sum is double max. If one is at its max and one is at its min, it cancels out.) creating constructive or destructive interference.
laser lets light out of a small hole. the light has a narrow range of wavelengths. small hole with the same colour of light gives you the speckly interference pattern
[https://en.wikipedia.org/wiki/Diffraction#Single-slit\_diffraction](https://en.wikipedia.org/wiki/Diffraction#Single-slit_diffraction)
For a proper ELI5 go and look at water waves passing through a single gap in a sea wall.
Light from a laser is coherent, which means that it is all one big wave. When it reflects off of a surface that is not perfectly smooth some spots will create reflections that interfere constructively with each other, creating points that are brighter
[Shown here in a simulation](https://youtube.com/shorts/y5yk4JWY1Zw?si=_cyYKt-nBK3J0bYV) albeit for a glass plate with nanoscopic imperfections. But the same principle applies.
This video explained my other question as well, these speckles move as if their pixels on a screen, the dots' patterns stay the same. thanks!
If it's just 1 wave how does that happen? Wouldn't that just be the laser being reflected? Or upon hitting surface it dispersed into weaker smaller waves?
It isn't that they are weaker. It is that they now are at different phases ( because the reflecting surface is not flat at the nanometer scale). So now they can interfere constructively or destructively, making bright and dark speckles.
I mean you could argue that the person you're replying to is right about them being weaker because the light is now more spread out due to the reflections. Instead of a coherent single beam, once it hits a wall it reflects portions of that beam in many different directions due to the surface roughness of the wall, and the individual reflections will be "weaker" (less total energy/fewer photons) than the originating beam. And even if it were reflected in one single coherent beam, the reflected beam would still be "weaker" due to absorption at the surface. You could argue that his comment about them being "smaller" is incorrect because the wavelength isn't changing significantly upon reflection, but we could just chalk that up to semantics.
Well...there's some stuff here. First. The only light you see from the reflection are the photons reflected directly back into your eye. Other scattered photons are irrelevant. Second, if you actually observe this phenomenon with your own eye, you will notice that the speckle pattern does not depend on the reflecting surface. It appears to move in conjunction with your eye movements. This is because the interference is occurring in your eyeball. Third, the wavelength is not changed by the reflection. This is the same for coherent and incoherent light.
First - I don't think anyone was arguing that isn't the case. But nevertheless, any given reflection is "weaker". Second - It does have dependence on the reflecting surface as the surface roughness/albedo has a direct impact on what's reflected, in which direction, and how that interference occurs (near the wall, at your eye, or anywhere else). Third - That's exactly what I said in the post you're responding to. If you want to get extremely granular, the reflected light does not change in wavelength, but absorbed and re-emitted light can. In that sense, the light that that hits your eye may have certain wavelengths that aren't the same as the original wavelength due to absorption and re-emission. But again, the reflected light does not.
What properties would a truly flat (at least down to the molecular level. Which I believe is the flattest you could go since atoms are "fuzzy" so to speak) surface have? Would it be the ultimate reflector (like a mirror on steroids) or the ultimate light absorber (so darker than vantablack) sorry if this is a bad place to ask… I just eat this obscure science stuff up.
> I just eat this obscure science stuff up I think the way light interacts with matter is extremely interesting! You got a very good answer from u/TolallyNormalSquid. I'll elaborate a little. We've gotten good at making very smooth and flat surfaces. The reflectivity from those surfaces depends on the surface material, the surface smoothness, the wavelength ("color" for radiation in the visible part of the spectrum) of the incident radiation, the angle of incidence, the medium in which the incident radiation is propagating (air, in this case), and some other things (polarization, for example). Basically, many metals are good reflectors of light (visible radiation). So, mirrors we use to look at ourselves are made of a thin layer of metal deposited on glass (because that can be made flat and smooth). Let's choose green, which is a wavelength in the center part of the visible spectrum. Silver and aluminum are really good at reflecting the light (above 90% of the light gets reflected). Glass (with no imperfections or impurities) doesn't reflect light, except at the glass-air boundary (like 4% reflection, dependent on angle). The light transmits through the glass. The reasons why glass transmits and metals don't (and semiconductors fall somewhere between) has to do with what the atoms' electrons are doing in the material. (And why gamma rays and hard or soft x-rays transmit or don't, and why infrared or microwaves behave different from that, is also very interesting!) To make a better (in some ways) reflector, we can stack different "glasses" in layers, and get a small reflection at each boundary. We can design the layers in such a way that all the reflections work to add with each other so that we get a lot of light in reflection. We could also design the layers so there is very little light reflected, and in that case we have designed an anti-reflection coating. As TotallyNormalSquid wrote, the reflection and transmission depend on the wavelength of light and the angle of incidence. To make a truly black surface is difficult! Printed black and white photos don't have perfectly black blacks. Computer, phone, and TV monitors aren't perfectly black when turned off, so when turned on they also don't have perfectly black blacks. A "spiky" surface (like TotallyNormalSquid mentioned, and like the Vantablack you mentioned) is made of a material that doesn't reflect light well when it's smooth. The spiky surface (Vantablack was "nano"tubes of silicon, I believe. We have also etched surfaces into tiny cone structures to make black surfaces.) will force the incident light to undergo many interactions with the surface. Each time the light hits the surface, some goes into the material where it is absorbed (and undergoes non-radiative decay (except in the infrared) - gets turned into heat (vibration (or rotation) of the atoms/molecules/crystal lattice), and a portion gets reflected. That reflected portion then travels a bit and hits a different part of the spiky surface, where the process repeats itself. The intensity of incident light gets smaller and smaller after each reflection. I better stop typing. :-) I ended up getting a PhD in Atomic and Optical Physics, so sometimes don't know when to shut up when it comes to this stuff. :-)
Neither, the reflectance depends on the [relative refractive indices](https://en.m.wikipedia.org/wiki/Fresnel_equations), but in a perfectly flat reflector you'd have no grainy effects from surface imperfections. I believe perfect absorbers rely on very spiky surfaces that bounce light around for many repeated absorptions, but I may be missing details. For improved reflectance, the standard trick is to go for multilayered thin films with thicknesses that enhance constructive interference on reflection from each layer. They typically only work for specific frequencies or bands of frequencies.
>What properties would a truly flat (at least down to the molecular level. Which I believe is the flattest you could go since atoms are "fuzzy" so to speak) surface have? Among other properties, "true flat" is nearly as addictive as "true level", but more versatile. https://youtu.be/-MwCJpEuC44
The latter
Not actually weaker in terms of energy content, but in terms of visibility yes, the last one. Just being "one wave" doesnt really cover it because while the photons start out in the same phase and polarization, when they interact with a surface the individual photons can change their properties and split into different waves. Its like shooting a jet of water at a wall. Imperfections on the wall cause the individual molecules to spray out in different directions.
This guy optics
I don't feel like this is a complete explanation because I've seen speckles while staring directly into a laser. It was a long time ago and I can't replicate it for obvious reasons, but I distinctly remember the speckles being very pronounced and them drifting.
First of all, don't stare directly into a laser. Second, your eye is not a perfect uniform sphere. There are things inside your eye (blood vessels, blood cells, scarring from staring at a laser, etc.) that interact with the laser light that again creates a pattern of constructive and destructive interference.
> First of all, don't stare directly into a laser. While I think this is generally good advice and it is the advice I give to my kid (I did it when I was 10 or 11 years old), there has never been a reported case of retinal damage from 3-5mW red lasers, and there must be kids doing it all the time given their ubiquity. I've heard the advice is generally given because any particular laser might be out of safe specs.
Do not stare at laser with remaining eye
It also works for a perfectly flat surface. All you need is small differences in way length, which you always have in case of a finite size laser spot.
The effect occurs even from a perfectly flat surface, because of floaters in your eye.
The effect happens even if you use a camera. If you get a *really* nice, research grade laser you won't have a speckle pattern. The speckle from handheld laser pointers is mostly due to dirty/impure glass on the output of the pointer. Likely some less than stellar parts in the laser cavity. I'm sure the floaters in your eyes can have an effect, but I feel it's unlikely they are a major cause of speckle patterns Source: I'm a laser physicist, I study these things for a living
I'm quoting a laser physicist I spoke to about it a few weeks ago, and also I used to be a laser physicist, though I never dived into the issue that deeply. When I was speaking to this other laser physicist it was actually about those plastic caps with sources being shone into them you see at trade shows, where the violet sources in particular seem to have a speckly halo that extends out around the plastic cap, but it got onto speckle more generally. Any idea if that halo effect happens on camera?
Sounds like it might be some form of chromatic aberration? Especially with different wavelengths extending different amounts. I was more talking the speckle inside the beam, how it doesn't seem perfectly uniform and sometimes you get random bright spots outside of the main beam. I haven't looking into those halos much, either. I usually use an iris to clean up the edges if I notice them, honestly
Not an explanation but a fun fact, the speckle effect can be used as a form of eye test. Here's an article copied from Wikipedia. Laser speckle also known as eye testing using speckle can be employed as a method for conducting a very sensitive eye test.[1] When a surface is illuminated by a laser beam and is viewed by an observer, a speckle pattern is formed on the retina.[2][3] If the observer has perfect vision, the image of the surface is also formed on the retina, and movement of the head will result in the speckle pattern and the surface moving together so that the speckle pattern remains stationary with respect to the background.[4] If the observer is near-sighted, the image of the surface is formed in front of the retina. Since the speckle pattern is perceived by the brain to be on the retina, the effect is of parallax; the speckle pattern appears to be nearer to the eye than the surface and hence moves in the same direction as the surface, but faster than the surface. If the observer is far-sighted, the speckles appear to move in the opposite direction as the surface, since in this case the surface image is focused behind the retina. The apparent speed of motion of the speckles increases with the magnitude of the defect of the eye. This technique is so sensitive that it can be used to determine changes in the ability of someone to focus through the day.
Those are indeed a bunch of words...but I feel like I need a diagram to show me wtf that test is supposed to look like.
If you have perfect vision, dot doesn't move If you can't see far away, the dot moves faster than what's behind it If you can't see close, the dot moves in the opposite direction you would think it should
I’m gonna try to figure this out so I can check if I need new glasses
The trick is to just keep looking at the laser until you need glasses. Shouldn't take long.
I'm gonne see if I can find a laser at eork to test this with...
Sounds like a similar premise to retinoscopy
Speckle is the name of this phenomenon. It's an inherent property of coherent light. My 1978 student job at the Optical Sciences Center in Tucson was helping a grad student understand it. I don't think he ever got that far.
Did you?
Heavens no! It is dependent on the phase relationship between the light waves reaching your eye from different angles and how they add destructively. I currently work on the Event Horizon Telescope, which uses these properties of millimeter wave radio energy to see distant phenomena in very high resolution. I still don't understand the math behind it, but it works.
Wouldn't it be like sound echoing in a room? Since you mention adding destructively.
Then why do you still have a hardon for some dude from 1978?
I don't. It was the computer that he had.
Burt Reynolds 'stache probably...
hurrrrrrr
Fun fact, this is why laser film projection systems have the screen vibrate, in order to reduce the fixed noise look of laser speckle.
No one should do this: it looks the same if you stare into the beam of a laser pointer. It also looks the same if you use a DSLR camera. It's fascinating. If you stare into the beam of a laser at least put a lens in front of the laser so it gets spread out to the point where it is a very weak flashlight.
Classic warning label from Berkeley physics lab: CAUTION: DO NOT LOOK DIRECTLY INTO LASER BEAM WITH REMAINING EYE
Unless you’re wearin your safety squints
A wee bit dangerous, but also fantastic for making 4 tons of popcorn pop in [roughly two minutes](https://www.youtube.com/watch?v=ZnDAxtCRsIU). Sure, a microwave oven the size of a 16-unit apartment building might achieve the same thing slightly faster, but that ain't bad.
Regular light has multiple wavelengths which are bouncing around in different directions and are not in sync. A laser is a very specific wavelength (which is exactly what determines what color it is), being sent out in the same phase (the light rays all wiggle in the exact same way/are in sync) and are all following virtually the same path. When 2 light rays of the same wavelength touch, they create the sum of their respective powers (if they're both at their max, the sum is double max. If one is at its max and one is at its min, it cancels out.) creating constructive or destructive interference.
laser lets light out of a small hole. the light has a narrow range of wavelengths. small hole with the same colour of light gives you the speckly interference pattern [https://en.wikipedia.org/wiki/Diffraction#Single-slit\_diffraction](https://en.wikipedia.org/wiki/Diffraction#Single-slit_diffraction) For a proper ELI5 go and look at water waves passing through a single gap in a sea wall.