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Specular and diffuse reflection 2

Specular and Diffuse Reflection 2. Created by Sal Khan.

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  • aqualine seed style avatar for user nlongchamps
    I'm still not quite sure why the specular reflection is white. Why was energy not absorbed from that light ray, returning white light, yet the diffused reflection returns red? Its seems both are reflections of the same light source and both types of rays would have certain wave lengths absorbed equally.
    (82 votes)
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    • blobby green style avatar for user waveman68
      Specular reflection occurs at the very surface of the apple. Often apples are waxed so that they are extra shiny (it makes the colors seem more intense). Just like a mirror or a shiny white light everything is reflected. White light is a mix of all colors, so all colors get reflected.

      Specular reflection is not always white - think of mirrors made of silver, gold and copper. Silver reflects all visible light more or less equally. Gold and copper do not. Pure gold absorbs blues and greens and has a yellow appearance. Copper absorbs yellow too, so that it has a red-orange appearance. This has to do with the way light interacts with the electrons of these metals. This wasn't properly understood until the 20th century.

      Diffuse reflection is a bit more complicated than Sal's easy to understand explanation. The apple appears red because of pigments that absorb the other colors. This happens below the surface where the pigments are. Depending on the surface (I assume this is true of apples - I haven't looked at them) the light is bounced back and force in between the layers, in this case in the apples skin. These multiple reflections make the light diffuse, because the light is reflected differently between each of the layers (e.g., closer to of further from the white flesh).
      (69 votes)
  • purple pi purple style avatar for user inventeredison1
    So, is what we see in our mirror a specular reflection of us that is a diffuse reflection of the light source?
    (39 votes)
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  • male robot hal style avatar for user The Last Guy
    Sal said that the apple was reflecting because it was waxed nicely, but whywould waxing change themake up of the surface of the apple? Wouldnt it still be an uneven surface regardless of the wax? Why does the wax increase the reflecton?
    (7 votes)
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  • male robot donald style avatar for user Ahnaf Abid Shamit
    maybe a little out of the box question.
    if light gets absorbed by the object then shouldn't the object gain mass as it is absorbing the light particles or is it that the light particles dont have mass as it is travelling at the speed of light ? quite confused here
    (12 votes)
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  • male robot donald style avatar for user Micah Alex
    Sal said that in diffused reflection some part of the light is absorbed by the apple. And since light is in effect just energy particles, does this mean that the apple keeps more getting energetic the more you keep it in the light?(given that it isn't decaying)
    (6 votes)
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  • duskpin ultimate style avatar for user Divneet
    this question has been asked earlier in the first video of specular and diffuse reflection. the question was that during diffuse reflection will the angle of incidence be equal to angle of refection?
    i am still not clear because there were 3 different answers to the question which still makes it unclear.
    hence according to me the angles should be equal because its the normal's function to divide the angle and even the law. but my book says that law is only obeyed in smooth surface. thus this makes it unclear . so can someone please answer.
    (4 votes)
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    • leafers seedling style avatar for user SaketVaibhav
      the angle of reflection is in fact equal to the angle of incidence at all times. It does not matter whether the surface is smooth or uneven. When you come to think of it , the particles of light in the incident ray fall only on a minute point so when it gets reflected you take it as a plane surface and therefore the normal is always perpendicular to the surface ( which is a minute point).

      hope this cleared our doubt :)
      (4 votes)
  • blobby green style avatar for user mvsaditya
    is the specular reflection caused due to total internal reflection??
    (3 votes)
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    • piceratops ultimate style avatar for user teodoro
      Specular reflection is the mirror-like reflection of light (or of other kinds of wave) from a surface, in which light from a single incoming direction (a ray) is reflected into a single outgoing direction. Such behavior is described by the law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal, thus the angle of incidence equals the angle of reflection ( in the figure), and that the incident, normal, and reflected directions are coplanar. This behavior was first discovered through careful observation and measurement by Hero of Alexandria (AD c. 10–70
      (3 votes)
  • leaf red style avatar for user djungari sunfire
    how do things absorb (or reflect) certain wavelengths and not others?
    (2 votes)
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  • leaf green style avatar for user Jibran
    how will the apple look if its in a completely dark room and then green light is flashed at it. (Any light color that is not red)
    (2 votes)
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  • marcimus pink style avatar for user Ben Kazemi
    So, at around is he basically saying that all specular reflection - the white light we see on shiny surfaces is reflecting at an angle directly to the eye ?
    (1 vote)
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    • piceratops tree style avatar for user heba
      yes , he is saying that if the angle of incidence equals the angle of reflection at a point , and from this point the reflected ray came to our eye then we're going capture some information of the image ((usually the image of the light source eg :sun , bulb , etc)), thus it is a specular reflection
      (5 votes)

Video transcript

I wanna talk a little bit more about reflection and specular and diffuse reflection in particular and to start of, I wanna make a minor correction to the last video, and it's actually a pretty interesting correction because in this image right here I talked about a double reflection I talked about the light from the sun hitting this pond or specularly reflecting off of the surface of this pond and than reflecting off of this paddle to get to our eye and when I said where the light ray was passing in the last video, I said "it goes from the sun to this point-to this point on the surface cause that's where we saw the reflection and I said, "than from that point to this point of the paddle than to our eye that the correction here --and someone pointed this out on the message board and it's a very good catch-- is that this point is where the light if reflecting to our eye so that because we see it, we see the sun there so this light ray right here is coming directly to, directly to someone's eye-let me draw someone's eye right there or our eye, our eye coming straight out of the page the light that got reflected here is going to be reflected a little bit more shallowly so it came from the sun, it came from the sun now obviously this won't be exact, but it came from the sun it probably had a slightly more shallow (uh uh) angle of incidence or angle between the ray and the surface and than it would have gotten to than it would have gotten to the paddle than it would have gotten to the paddle, than it would get to our eye and I wanna make clear we don't see the sun at the point that it is reflected because that light that image of the sun is going to the paddle which than goes to our eye the point where we see the sun-the reflection of the sun that light's going straight into our eye it wouldn't have to be reflected by the paddle so that was an interesting and good catch when you think about the double reflection the other thing I wanna talk about is when you have both specular and diffuse reflection happening at the same time now, we see this all over the place any time you see kind of a shiny object something that's not a mirror, but it's a shiny object and we see that right here in the apple, just to make this clear so you can see, this-this- white part right here, which we've all seen on a nicely waxed apple this is, it's like a reflection of the actually light source while everywhere else, it's just kind of the color of the apple let's think about what's happening here so if I were to draw the surface, if I were to draw the surface of the apple let me draw it, let me draw it in a neutral color so let's say we have the surface of the apple right over here we have the surface of the apple right over there and than maybe I'll draw it as a slight circle, just so you can appreciate that it's a circular apple so the surface of the apple right over here you have some light source, so you have some light source ,so if you have some light source over here and it's, and it's emitting light so it is emitting light in some direction, onto the surface of the apple and what's happening on the surface of the apple, there's there's a couple of things that's happening you're having some of this light is reflecting specularly or, another way of thinking about it is it's maintaining the image, a little of it, but not all of it so let me just draw it, so if you're coming in at that incident angle you're going to leave at that reflected angle and if you came in at this incident angle, you're going to leave at that reflected angle and if you came in at this incident angle, going to leave at that reflected angle and I'm doing dotted lines to show that it's not all of the light that's getting reflected that way only some of the light is getting that specular reflection ,but if someone, if this light were to be reaching your eye if, let me draw a proper face, so let's say this is someone's face right here that someone's, that's there ear, or there hair that's there eyebrows, that's there moustacche and, I think you get the general idea so this person will see a reasonably good version of the light source which we see right over here, but what's going on the rest of the apple or even over here, so some of the light is reflecting specularly it's kind of maintaining the image of the light source although that's getting distorted a little bit but the rest of the light, the rest of the light and let me be clear here- the rest of-some of it is being absorbed some of it is being absorbed by the actual surface of the apple but the rest of it reflecting red light, what we see is actually what the light, apple does not absorb what the apple is reflecting, but that red light that mainly red I mean it's a bunch of different shades over here is reflecting diffusely, so that pure light comes in here, some of the pure light get reflected specularly or kind of purely maintaining the image, but some of it, because the surface of the apple isn't smooth is going in all sorts of directions, so of it's going like that, some of it's going like that some of it's going like that, and that's happening at every at every point on the apple, so that light source let me draw more of the light source, so rays coming in, and rays coming in from our light source but every point of the apple is also having this diffuse reflection which is reflecting only the red part of the spectrum, it's absorbing all of the other parts of the spectrum and, so what you see is, that point where you're getting the pure specular the pure specular reflections, that's the point that looks shiny where you can kind of see the light source and from every other point on the apple, where only this guy right here's only getting the diffuse red light and that's why, if you move the apple, or actually even better, if you move where you're looking at the apple that, point of reflection's gonna look, like if I start looking at it from this direction I'm gonna get a different point of reflection, cause I'm gonna start seeing the light though let's see, let's say that someone's eyeball is right, actually right over here so that's their eyebrows, that's their eyeball, that's their nose, you get the general idea so let's say that this person, let's say that this person, that's their ear there, I don't wanna get too involved in the drawing so let's say that this person is looking at this supper ginormous apple now relatively to him he is not gonna see this white spot there he is gonna see the white spot on the point where it looks like where he's getting the right angle, where, where the rays are coming in let me do it in a different color so let's say, let's say the light source is above us so the light source is coming straight down is coming straight down to the apple and than if it's the right angle if the angle of incidence and the angle of reflection relative to the surface of the apple and we can draw that out, we can draw it perpendicular to the surface of the apple this is the angle of incidence, this is the angle of reflection so those points, at that point right there he's getting a little bit and I should draw a dotted line he's getting a little bit of the, of the specular reflection, but from every other point so for every other point on the apple, so over here, the specular reflection would go that way not to his eyes, or it would go that way, not to his eyes but from those points you also have some of the diffuse reflections and some of that is going to his eye in terms of red the red part of the frequency is getting to his eye from all of these other random directions and that's why this guy over here, he would see this, this reflection he would see this reflection rougly over here, and all of this other part of the apple would look red and we, since we're on this side of the apple, we see the reflection point the light is coming straight down from our light source bouncing right here than coming straight out towards our eye coming down, and than reflecting, coming straight out towards our eye it's also coming down on these other points, but those other points are going straight out to some place where our eye isn't, instead we're just seeing, we're just seeing the diffuse part that is actually coming to our eye anyway, hopefully you found that a little bit more, um, filled in a couple of the gaps of the last video