Class 10 Physics (India)
Laws of reflection
In this video, we will explore what diffused (scattered) & specular (regular) reflections are, and also learn the rules governing the reflection of light. Created by Sal Khan.
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- Light can travel through space, but why can't sound?(7 votes)
- as light can travel in any medium, so it can travel through space. but sound needs a medium to propagate and it needs a medium like air to travel further as the air particles move in a wave when sound travels. if there are no molecules in space, the sound won't travel.(5 votes)
- Our book tells us that even for irregular surfaces,
while the video says the opposite (at around
i = r7:27). Which is correct here?(5 votes)
i = r.
This law always holds true. But the angles are dependent on the situation of the normal - the line perpendicular to the surface. In irregular surfaces, as the surface is knobbly, the normal points a different way at every point. So even though
i = r, it doesn't always seem like it.(6 votes)
- if the mountain diffused light, Why would all the light necessarily fall on water ? some light may hit the ground also and we would not see a complete image in the water . But how do we get a complete image then??(4 votes)
- The fact that mountains diffuse light is the reason why we can see it. The reflected rays from the mountain are directed in all directions. Some of these fall on the land, some towards the sky, some on the water surface. The light rays that reflect towards our eyes enable us to see the mountain. However, the reflected rays that are directed towards the surface of the lake do not reach our eyes directly. They reflect specularly off the surface of the lake and then reach our eyes, hence we see the image of the mountain in the lake. But the surface of land is not smooth, so there, once again, diffused reflection takes place and don't get any image.(6 votes)
- Why do we draw a normal from the point of incidence? Dont it essentially means that angle of incidence is imaginary?(3 votes)
- A ray of light is incident on a totally reflecting surface at a certain point. This point is called point of incidence.
If we draw a line which is perpendicular to the reflecting surface at the point of incidence, this line is called the normal i.e., normal is an imaginary line perpendicular to the surface reflecting the light.
Angle between the incident ray and the normal is called angle of incidence. Angle between the reflected ray and the normal is called angle of reflection.
The law of reflection states that when a ray of light is reflected off a surface, the angle of incidence is equal to the angle of reflection.
I think that the angles of incidence and reflection are imaginary ones. They help us understand things better. Perhaps it’s because it’s easier to visualize a reflection about that line rather than a symmetry with regard to the tangent to the surface. It’s mostly just a convention.
Pls do point out if I'm wrong.
Hope this helps!(3 votes)
- Is the first law of reflection imaginary?(2 votes)
- There are two laws of reflection:
1) The incident ray, reflected ray and normal lie on the same plane.
2) Angle of incidence is equal to angle of reflection.
In case you are referring to the first law,to some extent yes it is imaginary because a plane is a human made concept ( does not have any physical existence) but it is nevertheless important. Take two pencils and hold them together by connecting their sharpened ends. The law tells us that if these two pencils are light rays, they can only exist in a 'V' format.The normal would be lying 90 degrees to the surface. If you try moving one pencil forward or backward, notice that all three( incident ray, normal, and
reflected ray) can never be in the same plane.
If you are talking about law 2, then it is more practical. It is also important because it gives us a basic foundation of light's nature that angle of incidence is always equal to angle of reflection.(4 votes)
- Why do we prefer convex mirror in the cars and other vehicles?(2 votes)
- Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
Hope this will help.(4 votes)
- what is the meaning of sign convention(2 votes)
- Sign conventions are some 'convetional rules' which are used to know and decide the positions of objects and images with the help of + and - signs.
If you don't know about the notion of conventions then I can explain it to you as follows:
You might surely know about the number system- 0,1,2,3,4....-which is the English numereal system.
But there are numeral systems of other languages, like the hindi numeral system (though it is not used much substantially ,nowadays ). We have just accepted them without any scientific reason because we needed a common language to communicate with other people . Therefore, these numeral systmes are also conventional, and even languages thmeselves are conventional.(4 votes)
- Hi, can someone please answer my question:
How does light pass only through transparent and partially through translucent objects? does it have to do something with the inter molecular spaces of these objects? Thank you(2 votes)
- It has to do with the optical density of the material, and yes, this is dependent on the arrangement of molecules in the material. It is given by the formula n = c/v where c is speed of light in vacuum and v is the speed of light in the medium.. So rough things like paper which haphazardly reflect light and concrete, which is super thick appear opaque(3 votes)
- So, when the Sun reflects the reflecting is going everywhere but why??(2 votes)
- Off topic but what software is used to write here?(2 votes)
in this video we're gonna try to learn a little bit about a reflection or I guess you could say we are going to reflect on a reflection and I think most of us have a sense of what this is but we'll try to get a little bit more exact about it so there are actually two types of reflection and everything that reflects is doing one or the other or something in between so we have two types let me draw them so the first type and this is kind of what we normally associate this is what we normally associate with a reflection is specular reflection specular reflection and in specular reflection let's say that this is the top of a of a mirror this is the surface of a mirror if I have a light ray coming in so let me draw a light ray coming in and just to get the terminology right this light ray coming in this ray is the incident ray and it's the incident ray because it's the Ray as it approaches the reflective surface let me write that down that right there is the incident ray in incident incident ray it'll approach the surface and you can almost imagine that it bounces off at essentially the same angle but in the other direction so then it'll hit the surface and then it'll bounce off and it'll go just like that and then we would call this the reflected ray after it is kind of bounced off of the surface reflected a reflected ray and you may have already noticed this if you've played around a lot with mirrors you would see and and we're gonna look at some images so you can think about it a little better and next time in front of the bathroom mirror you can think about this and think about the angle of incidence and the angle of reflection but they're actually equal so let me define them right here so if I were to just drop a straight line that is that is at a 90 degree or that's perpendicular to the surface of the actual mirror right over here we would define this right here as the angle of incidence I'll just use theta that's just a fancy letter to show that the angle that which were coming in the angle between this ray and the vertical right there that's the angle of incidence and then the angle between that vertical and the right there and the blu-ray right there we call that the angle of reflection and it's just a property of especially mirrors when you're having specular reflection and you can see this for yourself and all the spit well at all of the you know regular mirrors that you might experience is that the angle of incidence is equal to the angle of reflection and actually we could see that in a couple of images over here so let me show you some images of specular reflection just to make it clear here so you have some light from the Sun hitting this mountain we're going to talk about diffuse reflection a little bit and that's what's happening it's being reflected diffusely that's why we don't see the actual image of the Sun here we just see the white but then that those white light rays and they're actually being scattered in every direction some of them are hitting the water so let's see this part I'm going to might try to match up parts of the mountain so you have this part of the mountain let me do this in a better color you have this part of the mountain up here and the part of the reflection right over there so what's happening right here is light is coming from that part of the mountain hitting this part of the surface of the water hitting let me see if I can draw this better it's hitting this part of the surface of the water and then it's getting reflected specular reflection to our eyes and it's actually coming straight at us but I'll draw it at a slight angle and then it's just coming straight to our and then it's coming straight to our eyes like this if our eye was let's say our eye was here it's actually coming straight out at us so actually should just draw a vertical line but hopefully this makes it clear and what we just said the angle of incidence is equal to the angle of reflection so if you were to draw a vertical if you were draw to vertical and it might not be that obvious here but this angle right over here let me draw this a little darker color this angle right over here that's the angle once it let me let me do that in a light color this right here is the incident angle we drew a vertical and the angle at which the light ray is approaching the surface of the water right before it bounces that's the incident angle relative to vertical and then this angle right here and I know it's hard it doesn't look like they're the same but that's just because of their perspective that we're dealing with this is the Aang of a reflection and they're actually going to be equal and you can also make a similar case and sometimes my brain has easier thinking about this if this angle is equal to that angle this is what's defined to be the angle of incidence and the angle of reflection we also know that this angle right here is going to be equal to that angle right there and my brain sometimes takes that because that kind of the angle between the Ray and the actual surface but they're really the same notion obviously it's a different angle but if this is equal to that then this is equal to that because these two are going to add to 90 these two are gonna add to 90 so another way you could view it is so if we look at the surface of the water the surface of the water is let me draw a line along the surface of the water another way to think about it if you can is that this angle this angle right over here is going to be the same as this angle right over there and you can also see it in this reflection right over here so the angle between the Sun so this the light from the Sun is going directly to the water here and then getting reflected at that point on the surface of the water and then coming over to our eyes and so we could either say that this angle is equal to this angle so the angle between the incident ray and the surface of the water is equal to the angle of the reflected ray and the surface of the water or we could draw a perpendicular right over here I'm not doing that too well we can draw a perpendicular right over here to the surface of the water and say that the incident angle the angle of incidence right there between the Ray and that perpendicular is going to be the same as the reflected angle as it's hard to see there once again because of the perspective but hopefully that starts to make sense and I encourage you go to your bathroom and look in the mirror and look at objects in the mirror and think about the angle that the light from the object must be hitting the mirror for it to get to your eye and where it's actually hitting the mirror it's actually pretty it's a pretty interesting thing to do if if you're looking for things to do in the in the bathroom now all we've talked about is specular reflection but the other type of reflection is diffuse reflection and this is the type of reflection that it may not be as obvious to you that it's occurring everywhere you look diffuse diffuse reflection and in diffuse reflection because the surface isn't a smooth it's not a kind of what we kind of associate as a mirrored surface so I'll draw it I'll zoom in a bunch so in diffuse reflection maybe the surface looks like that what happens is is and let me be clear in specular reflection any light ray that comes in like that the reflection will come off at the same the angle of incidence will always be equal to the angle of reflection this isn't a for the situation of say a mirror it'll always be the same if I come in at a steeper angle if I come in at a steeper angle then I'll go out at a steeper angle just like that that's for specular reflection for a diffuse reflection all sorts of crazy things happen and that's because we don't have this really smooth surface or the the molecules that make up the surface do crazy things to light so if I have if I come in in one direction right over here over at that point I might the light might reflect in that direction although if I come in at the same angle over here now all of a sudden the light might go in that direction and then if I come in at the same angle over here now the sudden the light might go in that direction and if I come in I think you get the general idea here if I come in over here now the light might scatter in that direction if I come in if I come in over here at the same angle now the light might scatter in that direction so the general idea is with diffuse reaction the the reflected rays are going in all sorts of crazy they're going in all sorts of crazy directions and they're getting all mixed up and so if you actually had an image here so if think about here if you had an image here of the Sun and I'm not drawing it in particular but let's say that these rays right here are coming from an image are coming from are coming directly from the Sun then when they reflect it'll kind of preserve the image you will have the reflected image of the Sun but over here if all of these light rays are coming from the Sun they're not all going off in the same direction this will be a part of the Sun part of the Sun it's happening at a really really small level so you're really just capturing the light but you're you're losing all of the information from the actual image and if you're wondering where diffuse reflection occurs just look around your room anything that's not a mirror is reflecting and diffusely it's diffusing the light you see that here the mountain right here is diffuse to reflection you have light coming from the you have light coming from the Sun but that's being reflected in all sorts of crazy directions in all sorts of crazy directions so you don't see a reflection of the sky over here the the water here that's specular reflection because it's so super smooth that it preserves the angle of incidence is going to be the angle of reflection it's always going to be the same angle because it's a kind of almost perfectly smooth surface the trees that's diffuse reflection and I also want to be clear on something like the tree so on on something that's white and white is the entire spectrum of light and we'll do more videos on that in the future it's reflecting the entire spectrum it's just mixing it all up so you don't see an actual reflection but if you look at the trees if you look at the trees you have the entire spectrum from the Sun coming down on the trees but the trees themselves and you should watch the videos on photosynthesis they're observed they're absorbing every other frequency of light except for the greens you see so they are just reflecting they're just reflecting the green they're just reflecting the green back to us and they're doing it in a way they're their diffusely reflecting it so we actually don't see an actual reflection in in those trees