0:01 in this video we're going to try to
0:04 learn a little bit about reflection
0:06 or i guess you could say we are going to
0:09 reflect on reflection i think most of us
0:11 have a sense of what this is but we'll
0:13 try to get a little bit more exact about
0:15 it so there are actually two types of
0:18 reflection and everything that reflects
0:21 is doing one or the other or something
0:22 in between so we have two types let me
0:25 draw them so the first type and this is
0:27 kind of what we normally associate this
0:28 is what we normally associate with
0:32 reflection is specular reflection specular
0:34 specular reflection
0:35 reflection
0:37 and in specular reflection let's say
0:39 that this is the top of a of a mirror
0:41 this is a surface of a mirror if i have
0:43 a light ray coming in so let me draw a
0:45 light ray coming in and just to get the
0:48 terminology right this light ray coming
0:51 in this ray is the incident ray and it's
0:53 the incident ray because it's the ray as
0:55 it approaches the reflective surface let
0:57 me write that down
1:00 that right there is the incident ray in
1:01 in incident
1:03 incident incident
1:04 incident
1:07 ray it will approach the surface and you
1:09 can almost imagine that it bounces off
1:12 at essentially the same angle but in the
1:14 other direction so then it'll hit the
1:16 surface and then it'll bounce off and
1:19 it'll go just like that and then we
1:21 would call this the reflected ray after
1:24 it is kind of bounced off of the surface reflected
1:26 reflected
1:28 reflected ray and you may have already
1:30 noticed this if you've played around a
1:33 lot with mirrors you would see
1:34 and and we're going to look at some
1:36 images so you can think about it a
1:37 little better next time in front of the
1:39 bathroom mirror you can think about this
1:41 and think about the angle of incidence
1:42 and the angle of reflection but they're
1:44 actually equal so let me define them
1:46 right here so if i were to just drop a
1:50 straight line that is that is at a 90
1:51 degree or that's perpendicular to the
1:54 surface of the actual mirror right over here
1:55 here
1:57 we would define this
1:59 right here as the angle of incidence
2:01 i'll just use theta that's just a fancy
2:03 letter to show that the angle at which
2:04 we're coming in the angle between this
2:07 ray and the vertical right there that's
2:10 the angle of incidence and then the
2:12 angle between that vertical and the blue
2:14 ray right there
2:17 and the blue ray right there we call
2:19 that the angle of reflection and it's
2:22 just a property of especially mirrors
2:23 when you're having specular reflection
2:25 and you can see this for yourself at all
2:28 the esp well at all of the
2:29 you know regular mirrors that you might
2:32 experience is that
2:35 the angle of incidence is equal to
2:36 the angle
2:39 of reflection and actually we could see
2:41 that in a couple of images over here so
2:43 let me show you some images of specular
2:46 reflection just to make it clear here
2:48 so you have some light from the sun
2:50 hitting this mountain we're going to
2:52 talk about diffuse reflection a little
2:53 bit and that's what's happening it's
2:55 being reflected diffusely that's why we
2:57 don't see the actual image of the sun
3:00 here we just see the white but then that
3:01 those white light rays and they're
3:02 actually being scattered in every
3:04 direction some of them are hitting the
3:06 water so let's see this part i'm going
3:08 to try to match up parts of the mountain
3:10 so you have this part of the mountain
3:12 let me just in a better color you have
3:14 this part of the mountain up here and
3:15 the part of the reflection right over there
3:16 there
3:19 so what's happening right here is light
3:21 is coming from that part of the mountain
3:23 hitting this part of the surface of the water
3:24 water
3:26 hitting let me see if i can draw this
3:28 better it's hitting this part of the
3:30 surface of the water and then it's
3:33 getting reflected specular reflection to
3:34 our eyes and it's actually coming
3:35 straight at us but i'll draw at a slight
3:38 angle and then it's just coming straight
3:39 to our
3:41 and then it's coming straight to our
3:43 eyes like this if our eye was let's say
3:44 our eye was here it's actually coming
3:46 straight out at us so i actually should
3:48 just draw a vertical line
3:50 but hopefully this makes it clear and
3:52 what we just said the angle of incidence
3:54 is equal to the angle of reflection so
3:55 if you were to draw a vertical
3:56 if you were to draw a vertical and it
3:58 might not be that obvious here but this
4:00 angle right over here let me draw this a
4:02 little darker color this angle right
4:04 over here that's the angle
4:07 let me let me do that in a light color
4:09 this right here is the incident angle we
4:11 drew a vertical
4:13 and the angle at which the light ray is
4:14 approaching the surface of the water
4:16 right before it bounces that's the
4:19 incident angle relative to vertical and
4:21 then this angle right here i know it's
4:22 hard it doesn't look like they're the
4:23 same but that's just because of their
4:25 perspective that we're dealing with this
4:28 is the angle of reflection and they're
4:30 actually going to be equal and you can
4:32 also make a similar case and sometimes
4:35 my brain has easier thinking about this
4:36 if this angle is equal to that angle
4:38 this is what's defined to be the angle
4:40 of incidence and the angle of reflection
4:44 we also know that this angle right here
4:46 is going to be equal to that angle right
4:48 there and my brain sometimes thinks that
4:49 because that's kind of the angle between
4:50 the ray and the actual surface but
4:52 they're really the same notions
4:54 obviously it's a different angle but if
4:55 this is equal to that then this is equal
4:57 to that because these two are going to
4:59 add to 90 these two are going to add to
5:01 90. so another way you could view it is
5:02 so if we look at the surface of the
5:04 water the surface of the water is let me
5:06 draw a line along the surface of the water
5:07 water
5:09 another way to think about it if you can
5:11 is that this angle
5:13 this angle right over here is going to
5:16 be the same as this angle
5:18 right over there and you can also see it
5:19 in this reflection right over here so
5:22 the angle between the sun so this in the
5:23 light from the sun is going directly to
5:25 the water here
5:27 and then getting reflected at that point
5:29 on the surface of the water and then
5:31 coming over to our eyes and so we could
5:33 either say that this angle is equal to
5:34 this angle so the angle between the
5:36 incident ray and the surface of the
5:38 water is equal to the angle of the
5:40 reflected ray and the surface of the
5:41 water or
5:44 we could draw a perpendicular right over here
5:45 here
5:47 i'm not doing that too well
5:48 we can draw a perpendicular right over
5:50 here to the surface of the water and say
5:52 that the incident angle the angle of
5:54 incidence right there between the ray
5:55 and that perpendicular is going to be
5:58 the same as the reflected angle as it's
6:00 hard to see there once again because of
6:01 the perspective but hopefully that
6:03 starts to make sense and i encourage you
6:05 go to your bathroom and look in the
6:06 mirror and look at objects in the mirror
6:09 and think about the angle that the light
6:10 from the object must be hitting the
6:12 mirror for it to get to your eye and
6:13 where it's actually hitting the mirror
6:15 it's actually pretty it's a pretty
6:16 interesting thing to do if you're
6:18 looking for things to do in the in the bathroom
6:19 bathroom
6:21 now all we've talked about is specular
6:22 reflection but the other type of
6:24 reflection is diffuse reflection and
6:26 this is the type of reflection that it
6:28 may not be as obvious to you that it's
6:30 occurring everywhere you look diffuse
6:35 reflection
6:37 and in diffuse reflection because the
6:39 surface isn't smooth it's not a kind of
6:42 what we kind of associate as a mirrored surface
6:44 surface
6:47 so i'll draw it i'll zoom in a bunch so
6:48 in diffuse reflection maybe the surface
6:51 looks like that what happens is
6:52 is and let me be clear in specular
6:54 reflection any light ray that comes in
6:55 like that
6:57 the reflection will come off at the same
6:59 the angle of incidence will always be
7:00 equal to the angle of reflection this is
7:03 in a for the situation of say a mirror
7:05 it'll always be the same if i come in at
7:06 a steeper angle
7:08 if i come in at a steeper angle then
7:10 i'll go out at a steeper angle just like
7:12 that that's for specular reflection for
7:14 diffuse reflection all sorts of crazy
7:16 things happen and that's because we
7:17 don't have this really smooth surface or
7:19 the the molecules that make up the
7:22 surface do crazy things to light
7:23 so if i have if i come in in one
7:26 direction right over here over at that
7:28 point i might the light might reflect in
7:31 that direction although if i come in at
7:33 the same angle over here
7:34 now all of a sudden the light might go
7:36 in that direction and then if i come in
7:37 at the same angle
7:39 over here now a sudden the light might
7:42 go in that direction and if i come in i
7:43 think you get the general idea here if i
7:45 come in over here now the light might
7:48 scatter in that direction if i come in
7:50 if i come in over here
7:52 at the same angle now the light might
7:53 scatter in that direction so the general
7:56 idea is with diffuse reaction the the
7:59 reflected rays are going in all sorts of
8:00 crazy they're going in all sorts of
8:02 crazy directions and they're getting all
8:05 mixed up and so if you actually had an
8:06 image here so think about here if you
8:08 had an image here of the sun
8:10 and i'm not drawing it in particular but
8:12 let's say that these rays right here are
8:15 coming from an image are coming from
8:17 are coming directly from the sun then
8:19 when they reflect
8:21 it'll kind of preserve the image you'll
8:23 have the reflected image of the sun but
8:26 over here if all of these light rays are
8:28 coming from the sun they're not all
8:30 going off in the same direction this
8:31 will be a part of the sun part of the
8:33 sun it's happening at a really really
8:34 small level so you're really just
8:37 capturing the light but you're losing
8:39 all of the information from the actual
8:41 image and if you're wondering where
8:42 diffuse reflection occurs just look
8:44 around your room anything that's not a
8:46 mirror is reflecting diffusely it's
8:48 diffusing the light you see that here
8:50 the mountain right here is diffused for
8:52 reflection you have light coming from
8:55 the sun you have light coming from the
8:56 sun but that's being reflected in all
8:59 sorts of crazy directions in all sorts
9:01 of crazy directions so you don't see a
9:04 reflection of the sky over here the the
9:06 water here that specular reflection
9:08 because it's so so super smooth that it
9:10 preserves it the angle of incidence is
9:12 going to be the angle of reflection it's
9:13 always going to be the same angle
9:15 because it's a kind of almost perfectly
9:18 smooth surface the trees that's diffuse
9:20 reflection i also want to be clear on
9:21 something like the tree so on on
9:23 something that's white
9:24 and white is the entire spectrum of
9:26 light and we'll do more videos on that
9:27 in the future
9:29 it's reflecting the entire spectrum it's
9:31 just mixing it all up so you don't see
9:32 an actual reflection but if you look at
9:35 the trees if you look at the trees you have
9:36 have
9:39 the entire spectrum from the sun coming
9:41 down on the trees but the trees
9:42 themselves and you should watch the
9:44 videos on photosynthesis they're
9:47 observed they're absorbing every other
9:50 frequency of light except for the greens
9:51 you see
9:53 so they're just reflecting they're just
9:55 reflecting the green
9:57 they're just reflecting the green back
9:59 to us and they're doing it in a way
10:01 they're they're diffusely reflecting it
10:04 so we actually don't see an actual
