Light Reflection: Understand How Light Interacts with Materials

[Raj Harsh]

Hello,

I have a question or perhaps a few questions regarding light and mirrors. When we point light at a mirror, it gets reflected back. But we can see the point light (assuming that it is a laser for the sake of simplicity) at a surface opposite to the reflective surface of the mirror. What piques my interest and leaves me perplexed is that the light is cast on the mirror, but it is also cast on another surface but the same is not true for things other than light ! In fact, I have conducted a few simple experiments to see the interaction of light with materials and how light sources act. For example, a fire does not act the same as a torch or a laser and it casts a shadow and a laser light is not reproduced or mirrored on another surface when we point at a reflective surface which is not mirror, for example, glass, even one with covered back or a metal like steel. We can argue that it depends on the light emitting radius to the body ratio but I think that becomes irrelevant at an angle. I would like to know what exactly is going on and how can light be duplicated or reproduced. Even if it loses it's intensity to abide by the law of conservation of energy, how exactly does this happen ? And why is it that other light emitting sources do not exhibit similar characteristics ?

Kindly provide me with a clear and an elaborate explanation.

Thank You.

 

[jedishrfu]

Can you elaborate more on what you mean?

Can you draw a diagram of your setup and show what you mean?

Light is light, mirrors reflect light. We have physical laws that describe the reflection and refraction of light so I don't understand what you're seeing that is contrary to these laws.

When reflecting light on a glass surface, some light reflects as with a mirror whereas some of it is refracted by the glass and comes out the other side.

 

[Raj Harsh]

Can you elaborate more on what you mean?

Can you draw a diagram of your setup and show what you mean?

Light is light, mirrors reflect light. We have physical laws that describe the reflection and refraction of light so I don't understand what you're seeing that is contrary to these laws.

When reflecting light on a glass surface, some light reflects as with a mirror whereas some of it is refracted by the glass and comes out the other side.

I do not think I would be able to draw a diagram to show what I mean but I will try my best to explain it with words.

What I mean is that we see a projection rather than a reflection when we use a laser pointer. Like I said, the results vary depending on the source of light. Fire emits lights but you do not see a projection of the light on, say, a wall. But when you use a laser or a torch, the light is actually replicated. I do not think it has much to do with the concentration of light because like you said, "light is light", the result should be the same regardless of the source, regardless of however many light rays or beams are there.

In simple words, what I mean to say is that it appears as if someone is shining a laser from the point on the mirror where the light is reflected or projected back from. But, if this is a property of light, an attribute, all of the light should share the same attributes. But it does not hold up as soon as you switch the reflective surface or the light source.

 

[mfb]

The light from the laser pointer is reflected like everything else. Lasers (and torches, just to a lesser amount) just emit the light more collimated - it all goes (nearly) in the same direction so it makes nice spots on walls. This is independent of mirrors.

 

[Raj Harsh]

The light from the laser pointer is reflected like everything else. Lasers (and torches, just to a lesser amount) just emit the light more collimated - it all goes (nearly) in the same direction so it makes nice spots on walls. This is independent of mirrors.

I do understand what you are saying and I do agree with you that collimation plays a vital role in that but like I said, regardless of the concentration of the beams at a point, it does not translate similarly or identically when the source of light is different, like fire, which forms a shadow. And like I told you, I conducted a few experiments of my own and each scenario yielded a different result. For example, using a shiny metallic object which is capable of reflecting light just as good as a mirror, does not give the same result as a mirror, i.e; the laser light is not projected back, or such was my observation. Another thing I observed is that when you angle a few mirrors to bounce back light from one another, when it's a point light, you will see it in the mirror. When you put an object in place of the laser, you will see the object as well but if you put a camera and use that as an observer, it will fail to capture either. The light for some reason may form a blurry radial gradient but the ball could not be seen. My experiments may not have been perfect and my measurements may not have been very accurate but I tried to be as precise as I could. While I did use small mirrors, I do not see how that will alter the results. I did these about an year ago and I have been researching ever since but I could not find any material to explain such phenomenon.

If I am not explaining myself clearly then let me know, I will try to do it better.

 

[mfb]

Fire itself doesn't form a shadow. Other objects can form a shadow when they block the light from fire. Same for the laser pointer and the torch. This shadow can be so large that it blocks the whole light - due to the collimation.

For example, using a shiny metallic object which is capable of reflecting light just as good as a mirror, does not give the same result as a mirror

Then it is not as good as a mirror.

When you put an object in place of the laser, you will see the object as well but if you put a camera and use that as an observer, it will fail to capture either.

The camera might struggle with low light conditions. A longer exposure could help.

 

[sophiecentaur]

@Raj Harsh : I think what you are seeing is the difference between specular reflection and diffuse reflection. The surface of a (good) mirror is totally flat (at a molecular level, sometimes) each part of the surface will reflect any 'ray' that hits it and sent it back at a symmetrical angle to the surface. Take a wide parallel beam of light and all the rays will be reflected in the same direction and the reflected image is complete and undistorted. If the beam hits a perfect reflector that is not flat but consists of many small surfaces, all at different angles, the rays will all bounce off in different directions. When you look at what comes back from a (matt) white surface you will see light from a wide range of angles as each ray goes off at a different angle. No coherent image is formed.
A good mirror has to satisfy two criteria. It must be flat and it must not absorb the light energy. Mirrors all use a metal surface for the reflection. This is at the back of the glass in a domestic mirror (for protection) but it is on the front surface for a reflecting telescope.

 

[russ_watters]

In simple words, what I mean to say is that it appears as if someone is shining a laser from the point on the mirror where the light is reflected or projected back from.

If you mean you can see the spot on the mirror where the laser its it, that's because the mirror isn't perfectly reflective and the laser is very bright.

If you are referring to seeing the beam in the air, the explanation is similar: the air isn't perfectly transparent and the laser is bright.

 

[Raj Harsh]

If you mean you can see the spot on the mirror where the laser its it, that's because the mirror isn't perfectly reflective and the laser is very bright.

Yes, that is what I was referring to.

 

[russ_watters]

Yes, that is what I was referring to.

Ok, then my answer and @sophiecentaur 's more detailed response apply.

 

[sophiecentaur]

If you are referring to seeing the beam in the air, the explanation is similar: the air isn't perfectly transparent and the laser is bright.

That reminded me of sitting in a cinema in the days when smoking was allowed. You could almost tell what was going on by looking at the visible beam from the projector going overhead. A small cinema in Paris with everyone puffing away on Gauloise Disque Blue. The good old days - not.

 

[Raj Harsh]

Thank you all for your insightful answers. I have narrowed it all down to two things :-
1. The mirrors are not as flat as they appear to be or we might think they are, even irregularities at the molecular level can lead to anomalies.
2. A camera needs to be made to operate like a human eye by adjusting it's settings such as exposure etc.

A question - How can I find out whether a mirror is perfectly flat or not ? Is there any possible way to polish it by myself ?

 

[sophiecentaur]

Yes, that is what I was referring to.

There could be two mechanisms to account for a visible spot on the mirror with a laser beam. The surface could have dirt and irregularities but also the glass (between the air and metal backing) could have imperfections (bubbles etc. ) which could also scatter the light and make the spot visible

 

[Raj Harsh]

There could be two mechanisms to account for a visible spot on the mirror with a laser beam. The surface could have dirt and irregularities but also the glass (between the air and metal backing) could have imperfections (bubbles etc. ) which could also scatter the light and make the spot visible

Thank you for the clarification. I have one more question, it is also regarding light in a way so I am not sure if I should start a new thread or just ask it here.

 

[sophiecentaur]

Thank you all for your insightful answers. I have narrowed it all down to two things :-
1. The mirrors are not as flat as they appear to be or we might think they are, even irregularities at the molecular level can lead to anomalies.
2. A camera needs to be made to operate like a human eye by adjusting it's settings such as exposure etc.

A question - How can I find out whether a mirror is perfectly or not ? Is there any possible way to polish it by myself ?

A good flat mirror will allow you to produce interference fringes with the aid of another 'optically flat' glass plate. Producing hi spec flat glass surfaces is very hard and a skilled process. Take two sheets of glass and polish them together - this will always cause one surface to become convex and the other surface concave. What you have to do is to use three sheets and polish different pairs in sequence. That prevents the curvature. (if you're good at it.)
This link will show you what I am talking about. Notice the interference fringes in the pictures of two surfaces brought together. If you look up Newton's Rings there are many hits and the rings can be obtained with a shallow curved glass, resting on a flat glass. But this is a bit of a diversion (interesting, nonetheless)

Photographing many optical phenomena can be quite hard - quite a bit harder than Aunty Doris at the sea side - I have tried.

 

[sophiecentaur]

Thank you for the clarification. I have one more question, it is also regarding light in a way so I am not sure if I should start a new thread or just ask it here.

Ask.

 

[Raj Harsh]

A good flat mirror will allow you to produce interference fringes with the aid of another 'optically flat' glass plate. Producing hi spec flat glass surfaces is very hard and a skilled process. Take two sheets of glass and polish them together - this will always cause one surface to become convex and the other surface concave. What you have to do is to use three sheets and polish different pairs in sequence. That prevents the curvature. (if you're good at it.)
This link will show you what I am talking about. Notice the interference fringes in the pictures of two surfaces brought together. If you look up Newton's Rings there are many hits and the rings can be obtained with a shallow curved glass, resting on a flat glass. But this is a bit of a diversion (interesting, nonetheless)

Photographing many optical phenomena can be quite hard - quite a bit harder than Aunty Doris at the sea side - I have tried.

Thank you so much for your help ! I will get onto reading that Wikipedia page but here is my another question, of a series of those. When exactly is an image formed ? And what is an 'image' ? I have read many definitions, the bookish kind, but I would like an intuitive one. What is I mean to ask is, what exactly is it that causes reflectivity ? If it is the light which when enters our eyes enable us to see, and what we see is the object which the light bounced from before reaching our eye, before the transduction process. If it were not for reflectivity, we would not have been able to see ourselves. What I have noticed is shiny objects tend to reflect back, even a polished wood will show some signs of specularity and reflection. Shiny is a word which can be used arbitrarily, but I think you understand what I mean here, referring to the reflectivity of a surface and not it's specularity. Materials play a huge role in this. For example, a well polished (perhaps a dark) leather will be a shiny object and in a way, reflect back light as is but it will not show any reflection in the form of an image. So, what exactly causes mirror images to form ? What exactly causes such a reflection ?

I ask this because, firstly, I have had this question for a long time. And the second reason is because of my first question, the observation and my belief of the occurrences, the incidents and what is actually happening. When light bounces from our an object and then hits the mirror and ricochets back, we see a virtual image. But, when we use light, when it gets deflected or reflected back, we see something I will consider to be a real image on another surface. Let us assume that it is a wall behind a laser pointer on which the light is cast, i.e; after being reflected back from the mirror. Do you understand what I am trying to say here ? If not, then inform me so that I can try to explain it differently.

All this becomes overwhelmingly complex when I start thinking about the absorption of light by mirror or the reflective surface etc. The intensity of the projected light (from the mirror) is less than the original source's. It almost appears as if a new source of light has emerged. Then there is this question about transduction. I did study Biology for an year but then I switched. What exactly do we see ? Is it the light that we see, which when enters our eyes forms an image ? Then why is it that direct light source is 'blinding' ? Why cannot our eyes interpret pure photons ? And regarding photons, it appears as if the light on the wall is the result of actual photons being projected, which would mean that the mirror is reproducing photons, instead of just reflecting them because if it were not, the mirror would have cast an image on the wall of the object on the wall just as it did with the light, but that does not happen.

 

[russ_watters]

...
All this becomes overwhelmingly complex when I start thinking about the absorption of light by mirror or the reflective etc...

It feels like you are making this way more complicated than it needs to be. So a few simple principles to apply:

1. At a basic level, we only see photons, and only when they hit our eyes (or, rather, our retina).
2. The information the photons carry (the image) is determined in part by how the photons are organized/arranged (there is also intensity and color).
3. There is a limit to how intense of a light our eyes can detect without being oversaturated.
4. Nothing is perfectly specularly reflective. The better specularly reflective something is, the better the image it provides you.

Much of what you want to know about the nuts and bolts of reflection is here: https://en.wikipedia.org/wiki/Reflection_(physics)

 

[russ_watters]

A question - How can I find out whether a mirror is perfectly or not ? Is there any possible way to polish it by myself ?

What is your goal here? Are you trying to make a mirror that is good enough that you can't see the laser spot on it? Why? And is that really going to be useful if you still can see the beams due to the dust in the air?

 

[Raj Harsh]

It feels like you are making this way more complicated than it needs to be. So a few simple principles to apply:

1. At a basic level, we only see photons, and only when they hit our eyes (or, rather, our retina).
2. The information the photons carry (the image) is determined in part by how the photons are organized/arranged (there is also intensity and color).
3. There is a limit to how intense of a light our eyes can detect without being oversaturated.
4. Nothing is perfectly specularly reflective. The better specularly reflective something is, the better the image it provides you.

Much of what you want to know about the nuts and bolts of reflection is here: https://en.wikipedia.org/wiki/Reflection_(physics)

Sir, I think my concern is regarding the image formed on the non-reflective surface, which in this case, is represented by a wall. You can see the spot on the wall, so an image is formed. It were as if someone was simply flashing a light at the wall, the only difference being that the intensity seems to be less. It is the same as when a projector projects film on a plotter. Does this make my question clear ? And now, the second part is that regardless of how the light reaches the wall, light is light, photons are photons. Then why is it that an object which does not emanate or produce light on it's own cannot be seen on the wall if the photons bounce from the object, head towards the mirror and then reaches the wall ? In both cases, it is light that reaches the wall then why are the results so different ?

What is your goal here? Are you trying to make a mirror that is good enough that you can't see the laser spot on it? Why? And is that really going to be useful if you still can see the beams due to the dust in the air?

There are plenty of reasons for this. If there is no spot on the surface, such a material would be perfect for screens of/on electronic devices, it will be the perfect anti-glare screen. But thinking of it makes me wonder that if all the light gets reflected back, whether or not it can also act as a good reflector, and perhaps a better and cooler (literally) solar reflector.

The beams due to the dust in the air is not much of my concern. But it is a good thing that you brought it up as I had an idea for optic cable without reflective inside, which I had long before. Many of my questions come when I start thinking of different ideas. We can perhaps fill up the inside of the tube with gas, more like smoke but not as dense, and use that for transfer of light. Then, it will only be a simple point-to-point connection, effectively reducing the cost as well. Both the terminals will have a light, a nozzle for control over the flow of gas and a few more things but not very much.

Another idea I had was to make automatic adjusting wheels which would move up and down to make rides smoother. This idea is a culmination of me trying to think of the possibility of such while trying to link different phenomena which would make this possible. For example, a toy car's wheels become loose and they start going up and down. If the wheels were to go up and down, we will not feel jerks in car. There of course, will have to be a limit to how far the wheels can move but even if they moved a little, it will help reduce the jerk. We do have hydraulic spring in our cars but those shockers are not as effective. The upper body of the car will stay stationary if the wheels adapted according to the surface which it would be in contact with, thus stabilizing the car. We can connect the front two wheels and the two in the back together but individual control would be better as because when one wheel would go up, it will not make the other wheel go up, otherwise it may not be as stable. But there indeed has to be limitation so that the car does not flip. It does not have to be a relay type system. I am from India and the roads here are not as smooth as one would want them to be and so, I had planned to work on this but it seems like a far-fetched dream. I am aware that it is not as simple as it sounds but I do believe this can be done. And sorry for going off-topic.

 

[sophiecentaur]

@Raj Harsh I think you are asking too many questions and bringing too many ideas into each of your posts. I suggest you ask one question at a time. That way it is easier for PF to answer you. One thread at a time and wait until one topic has been sorted out till you come up with the next one.

 

[DaveC426913]

Fire itself doesn't form a shadow.

Nitpick: fire does indeed cast a shadow.
Fire is not entirely transparent to light, as it contains roiling gases, as well as partially combusted products, and these will block (or deflect) a small amount of light from other sources. It's often not visible because the flame is usually the closest source of light to the surface and the shadow is washed out.

 

[DaveC426913]

Then why is it that an object which does not emanate or produce light on it's own cannot be seen on the wall if the photons bounce from the object, head towards the mirror and then reaches the wall ? In both cases, it is light that reaches the wall then why are the results so different ?

You do get light reflecting off non-emitting objects onto walls.

Take a selfie.
Grab a large sheet of day-glo green posterboard from your local craft shop.
Hold the poster-board horizontally out in front of you, like you're serving a dinner plate.
Take another selfie.
You will most definitely be hued in garish green tones - as will the walls around you.

The reason it doesn't appear the same as a laser is because the reflected light from the poster board is scattered in all directions, not collimated like the laser.

Normal folk often don't notice this reflected light, but you can bet every photographer and painter see it and uses it abundantly in their work.

This is the effect:

(except it's reversed: the ground is the object reflecting its light, while the egg is serves as the "wall" that catches the light)

This the artist using that effect to his advantage:

Here is a photographer using his own body as a reflective panel to cast various colours on his subject:

"WHAT YOU WEAR AS A PORTRAIT PHOTOGRAPHER MATTERS MORE THAN YOU REALISE"

 

[Raj Harsh]

You do get light reflecting off non-emitting objects onto walls.

Take a selfie.
Grab a large sheet of day-glo green posterboard from your local craft shop.
Hold the poster-board horizontally out in front of you, like you're serving a dinner plate.
Take another selfie.
You will most definitely be hued in garish green tones - as will the walls around you.

The reason it doesn't appear the same as a laser is because the reflected light from the poster board is scattered in all directions, not collimated like the laser.

Normal folk often don't notice this reflected light, but you can bet every photographer and painter see it and uses it abundantly in their work.

This is the effect:

(except it's reversed: the ground is the object reflecting its light, while the egg is serves as the "wall" that catches the light)
View attachment 226763
This the artist using that effect to his advantage:
View attachment 226764

Here is a photographer using his own body as a reflective panel to cast various colours on his subject:
View attachment 226765
"WHAT YOU WEAR AS A PORTRAIT PHOTOGRAPHER MATTERS MORE THAN YOU REALISE"

Thank you Sir for such clear explanation. But I think you may have misunderstood my question, or perhaps I failed to ask it properly. Nevertheless, I shall provide some clarification. I used the word reflection in context to image formation, or my image formation statement. So what I was asking had little to do with the light reflected from the object only, but more to do with the light being reflected back from the mirror and being projected onto the wall. Like I said before, a seemingly real image would form on the wall, whereas the same does not happen in the other cases. I am not referring to the carriage of colour but the entire image itself. I am speaking of image formation.

I was reluctant to say or mention this because I afraid it would sound stupid and I have not found any article on such a phenomenon but I think I should tell it. Interestingly, at my maternal grandmother's house, there is a little hole in the window pane which happened during the fitting of the A.C. but the great thing about it is that some light leaks through and so, images of whatever is on the outside, forms on the ceiling. For example, if a vehicle was passing by, you would see it on the wall, with colours ! And this is true ! This is related and connected to my question.
And the example above of the photographer and the colour of his clothes is more about illumination than it is about pure image formation.

And would you please try answering my second question about the reason of specularity, reflection ? What causes it to happen ?

 

[sophiecentaur]

And would you please try answering my second question about the reason of specularity, reflection ? What causes it to happen ?

Try reading post #7 again. To form an image in any way, the rays of light entering your eye have to be coming from the same point on the original object need to come from, or appear to come from, the same point, somewhere else. That is what makes an coherent image. That can only happen when all the rays from any particular point the object follow very similar paths.
Google "Optical Image Formation" or similar terms. There will be hundreds of suitable hits.

I think you have fallen into the (common) trap of trying to learn about Physics just by question and answer. That method is fraught with problems because you cannot be sure that the questions are the best ones to ask and that they are in the 'right' order. You may feel too impatient to flog through a proper Physics course and you may reply to my comment that it is just not your way of learning. However, I think many of the PF members who have been answering your questions would agree with me.

 

[mfb]

Interestingly, at my maternal grandmother's house, there is a little hole in the window pane which happened during the fitting of the A.C. but the great thing about it is that some light leaks through and so, images of whatever is on the outside, forms on the ceiling. For example, if a vehicle was passing by, you would see it on the wall, with colours !

This is called camera obscura and it was one of the earliest methods to make a camera. It has nothing to do with mirrors, however.

 

[sophiecentaur]

This is called camera obscura and it was one of the earliest methods to make a camera. It has nothing to do with mirrors, however.

Also look up Pinhole Camera. I remember seeing an image of the Sun, complete with a huge sunspot, formed as light passed through a keyhole in a back bedroom door and fell on the white panel of the front door.