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Physics library
Course: Physics library > Unit 15
Lesson 1: Reflection and refractionSpecular and diffuse reflection
Specular reflection, which occurs with smooth surfaces like mirrors, causes light rays to reflect at the same angle as they hit the surface. Diffuse reflection, which occurs with rougher surfaces, scatters light rays in different directions. The video also introduces double reflection, which occurs when light reflects off of one surface and then reflects off of another surface before reaching your eye.
Created by Sal Khan.
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- If build a box with mirrors, all on the inside, so the entire inside of the box is all mirrors, and will all give off specular reflections, and then cut a circular hole in the top and insert an ON light bulb,
will the light level keep on building up? Or are the photons adsorbed or "die" somehow?
I understand that the actual light bulb may absorb some of the light, with a diffuse reflection.
But hypothetically, if the entire light bulb had a specular reflection and it was completely sealed, would the light keep bouncing around the box, even after the light is turned off?(104 votes)- That's a cool question.
I think that in real life, light will always find a way out in form of heat or something, but hypothetically it would be cool if we could lock light in a box :p(18 votes)
- When i look at a light source and close my eyes immediately, why does it appear green?(30 votes)
- In our eyes there are microscopic tiny sensors which are sensitive to light.They have 3 colors red,blue and green.If you look at a very bright thing such as the sun and close your eyes immediately you will see red.This means that red is the most sensitive to light.When you see any light source glowing brightly and when you see it and close your eyes immediately you will see green.In your case you might have seen a brightly glowing bulb.So green is less sensitive to light than red.Finally if you see a light source glowing dimly you will see blue color.So blue is least sensitive to light.One interesting fact for you that when you look at the sun immediately and close your eyes firstly you will see red color.But as the light intensity seen by our eyes decreases you will see green color then and finally you will see blue color and then you will see only black.The order of sensitivity of the receptors are:
Red > Green > Blue
Red -----------> Most Sensitive
Green------------> Less Sensitive
Blue------------> Least Sensitive
Hope you like it(92 votes)
- Why do we see images of us in glass at times ?(10 votes)
- That's because glass reflects light (specular reflection). But it only reflects a fraction of it. Most of the light goes through glass.(24 votes)
- Is there any proof for why angle of incidence is equal to the angle of reflection as far as specular reflection is concerned?(10 votes)
- An intuitive proof is very easy if you assume that all the basic laws of physics stay the same if you switch the direction of time (at least of classical physics, and let's disregard thermodynamics for a second).
Take a film of two billard balls colliding. Now play it backwards. There is no way to tell, which direction actually happened.
Therefore the incoming and the outcoming angle of a beam of light have to be the same.(19 votes)
- if in diffused reflection light scatters away then we must not be able to see it . then how on the world are we seeing all this objects ?(5 votes)
- Keep in mind that the diagram he made is a microscopic view of an object on the atomic level. There are billions of light rays and billions of surfaces. So while you see things, a lot of the light is diffused. That's why when you look at something, it isn't like looking into the sun, but you still get enough light to see it.(14 votes)
- On a micro level, is all reflection specular, with the angles the same as they reflect off the various surfaces, and the diffuse reflection is caused by inconsistent surfaces, or does some light not reflect at an angle equal to the angle it hit the surface at?(9 votes)
- YES! All light reflects at equal angles of incidence and reflection. It is the changing orientation of the normal on certain surfaces that are not smooth (like the paint on your wall) that causes this shift, and diffuse reflection occurs. Diffuse reflection refers more to the collection of the reflected rays. If the incident rays are parallel, but the reflected rays are not, diffuse reflection occurred.(2 votes)
- Am I right in saying that the light would be continuously reflected forever? Would it ever stop reflecting or would it just carry on for ever?(3 votes)
- each time light reflects, a little is absorbed or refracted so it will eventually all be gone
ok??(5 votes)
- I am confused on the difference between diffused and specular, if you consider...
consider a round / unsmooth object. If you zoom in really far -further and further, eventually you will find a point where it looks like a geometric shape with multiple sides (think of a circle on the computer -if you look hard enough the pixels are still square at the edge), right? So wouldn't diffused reflection just be specular but bouncing on many surfaces?
See the image I made below because just words is too confusing:
http://imgur.com/je1xdIQ(5 votes) - Why is the angle of incidence the same as the angle of reflection?
I don't understand why the two angles are the exactly the same. My teacher told us that it's like a game of pool.(3 votes)- Because of conservation of momentum.
Think about this: if the angle of reflection were going to be different from the angle of incidence, what would determine what the angle of reflection would be? Would it be bigger than angle of incidence? Smaller? Why?(2 votes)
- So does this mean that we never see the true colors of things because there's a reflection or a shadow on it?(3 votes)
Video transcript
In this video we're
going to try to learn a little bit about reflection. Or I guess you could say we are
going to reflect on reflection. 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 with reflection
is specular reflection. And in specular
reflection, let's say that this is the
top 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. 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 ray. And you may have
already noticed this if you've played around a lot
with mirrors you would see-- and we're going to
look at some images. So you can think about
it a little better. Next time you're 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 at a 90
degree, or that is 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 at which we're 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 blue 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 at all the 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. And we're going to talk
about diffuse reflection in 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 those white light
rays, and they're actually being scattered in every
direction, some of them are hitting the water. I'm going to 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. 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 eyes like this. If our eye was-- Let's
say our eye was here. It's actually coming straight
out at us so I 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, 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-- 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 the perspective that we're dealing with. This is the angle of reflection. And they're actually
going to be equal. And you could also
make a similar case. And sometimes my brain has
easier thinking about this. If this angle is
equal to that angle-- and 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
thinks that because that's kind of the angle between the
ray and the actual surface, but they're really
the same notions. And 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
going to add to 90. So another way you
could view it is-- So if we look at the
surface of the water. Let me draw a line along
the surface of the water. Another way to think about it
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 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. And 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 a pretty
interesting thing to do if you're looking for
things to do 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 reflection. And in diffuse reflection,
because the surface isn't smooth, it's not
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-- 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 is for the situation
of, say, a mirror. It'll always be the same. If I come in at a
steeper angle, then I'll go out in a steeper
angle, just like that. That's for specular reflection. For diffuse reflection all
sorts of crazy things happen. And that's because we don't
have this really smooth surface, or the molecules that
make up the surface do crazy things to light. So if I come in in one
direction, right over here, over at that point
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 all of a sudden the light
might go in that direction. And if I come in-- and 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 over here
at the same angle, now the light might
scatter in that direction. So the general idea is,
with diffuse reaction, the reflected rays are going in
all sorts of crazy directions, and they're getting
all mixed up. So 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 directly from the sun, then
when they reflect it'll kind of
preserve the image. You'll 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. And it's happening at a
really, really small level. So you're really just
capturing the light, but 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 diffusely. It's diffusing the light. Do you see that here? The mountain right here
is diffuse reflection. You have light
coming from the sun, but that's being reflected in
all sorts of crazy directions. So you don't see a reflection
of the sky over here. 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 trees. So 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, 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 absorbing every other frequency
of light except for the greens you see. So they are just reflecting
the green back to us. And they're doing it in
a way they're diffusely reflecting it. So we actually don't
see an actual reflection in those trees. And I'll just finish up
with one kind of neat thing because it's kind of
like playing billiards here, because you can actually
have a double reflection. That's why I even had
this image over here. The sun is being reflected
on the water here, and then you have a reflection. This is a direct
reflection of the sun, but this is a reflection
of the sun reflected on the water right over here. So what you have over
here is the light from the sun coming
to this reflection, reflecting at that
point on the water, then going to this
point on the paddle, and then coming to our eye. But once again the angle of
incidence in every situation here is going to be equal
to the angle of reflection. Although the paddle looks like
it's a little bit distorted here, so it's not a
completely smooth surface, so it makes the math
a little harder. But when you start
thinking about this it becomes pretty
interesting just to look at almost any
reflective surfaces and to think about
the actual angles.