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Virtual image
Virtual Image. Created by Sal Khan.
Video transcript
Let's say I've got some
type of a book over here. So there is my book. I'll write it down, book. Let me draw the entire book. So this is my entire
book, just like that. And then I have a
mirror over here. So this is a mirror. We're just looking at the
mirror from the side-- I can do a better job
of a straight line than that-- so that is a mirror. Now, what I want to do
is pick, and I'm really picking an arbitrary
point on this book. And we know that
there's light that is coming on this book--
maybe I haven't told us about the light source--
but we're assuming that the book will reflect
the light diffusely, like most books do. It is not a shiny book. And so any point on this book,
and we could pick any arbitrary point on this book, is
going to be emitting light in a whole bunch of directions. So what I'm just going to
do is pick two light rays that are being emitted from
this point on the book. It'll be more than
two, but this will help us understand
what's going on here. So let me pick
one light ray that goes like this-- it's obviously
not curving like that-- so it goes like this, and
hits the mirror. Incident angle is equal
to reflected angle, so it'll reflect off at the
same angle just like that. And let me pick another point
that is coming out radially from that same, or another ray
that is coming out radially from that same point, so
it'll come out like that. It has a larger
incident angle, so it'll have a larger reflected
angle, just like that. And now let's think
about what an observer, someone who has their
eyeball right over here-- so let's say someone has
their eyeball right over here. Let me draw their
eyebrows so that you get the general idea-- let's
think about what this person right over here
will actually see. And remember all of
reality is really just a big simulation
going on in our brains. Now, this person's eye is going
to see these two light rays diverging from
some common point. If you were to look
straight at the book-- let me just do another
example-- if someone were to just look at
any arbitrary object, let's say that's an
orange right over there. And just ignore all
the stuff on the left. If I were to look at an
orange, no mirror, nothing, no reflections,
or anything, if I were to look at a certain
point on the orange, light from that point would
be diverging out radially. And if my eye were
right over there, my brain says, OK, I
have these light rays diverging from a single point. It will simulate,
or it will show me that point of an
orange in my brain. Now, The same thing is
going to happen here, even though these
two rays actually aren't diverging from
a point out here. Your brain will say, OK, I got
two things that are coming out from what looks like
a point over here. It'll really just extrapolate
these lines backwards. So it'll take that
green ray and it'll say, OK, it must be coming
from a point out there. And it'll take that
magenta ray and it'll say, it must be coming from
a point out there. Your eye is just seeing
these two light rays, it doesn't know what's
going on over here. It just says OK, they're
just converging from a point. And so in this person's
reality, or in my reality, I think that there
is an orange there. Likewise, I see
these two light rays diverging from
some common point. I will extrapolate
to figure out-- or my brain will try to think
about-- where that point is in three dimensional space. It says, look,
these two rays look like they're coming from
a point back over here. Now, the reality is, there
is no back over here. There is nothing behind the
mirror, but to this observer, it looks like the light is being
emitted behind the observer. And so what the person
would actually see is something like this. And this is probably
not too foreign to any of our experiences. We've all looked in
mirrors-- and remember, this is the left side of
this book-- oh, sorry. This is the right
side of the book. It's the side that has
the curvy part of the B so it'll look like this. So what this observer sees is
an image that looks like this. It is what people
call the mirror image. It is flipped. The left and the
right is flipped and you can think
about why that is. I could just draw another ray
right here to make it clear. If this ray is coming like this. Now, it will reflect and
come out back like this to this observer it came from
that point right over there. So he sees a flipped
version of the book. Now the whole reason why I did
this is one, just to make you feel a little comfortable
with these reflecting lines and all of that. But one, I guess
more importantly to get you more comfortable with
the idea of a virtual image. This right here
is a virtual image and we're going to compare
it with a real image in the next video. And to some degree,
a virtual image is more intuitive because we
have so much experience with it when we think about mirrors
or reflective surfaces. So it's a virtual image. And we call it a virtual
image because the book really isn't there. It's virtually there. There is no actual
physical space, or we don't know if
there's any physical space behind the image. Our brain just uses
these diverging lights and creates a model
inside of our brain to say, OK, that
book exists there, even though the source of
the light is over here. Now I'm just going to
draw another diagram, and you normally won't
have the diagrams that have this type of
perspective, but just to give you a sense
of the same thing. And just to get you familiar
with some of these diagrams that we're going to do when
we study mirrors and lenses, is we could imagine
that this is the ground, let's say that this is a mirror. And let's say that
this is actually could be somebody standing
in front of a mirror. Although usually people
will draw an arrow here to just say an arbitrary object, Well, let's just say we have an
arrow in front of the mirror, just so you get used
to this notation. This is actually a physical
arrow, this is not a light ray. Now let's think about what
the image of this arrow-- let's say this arrow had eyes,
so it's actually a person. So this arrow had eyes,
what would this arrow see? Well, let's just pick an
arbitrary point of this arrow. You could do it with any
point-- this object-- because every point will be
reflecting light diffusely. So I'm going to take
one point that's coming straight out of the
mirror-- one light ray that goes straight to the mirror--
so just parallel to the ground, straight to the mirror. Well, it's just going to reflect
straight off and then come back to the source of that light. Maybe that arrow's eye, if
you view it as a person. And let's just do another
point, another ray coming from that point. So another ray coming from that
point might come out like that. Incident angle is equal
to reflected angle, so then it would
come back like that. Now, maybe this whole
thing is an eye. Maybe that'll simplify things. So let's say this
whole thing is you're looking really
close at a mirror. This whole thing is an eye. Now what would the eye see? Well, it's looking--
this ray looks like it's coming
from back over here. And then this ray
looks like it's coming from back over here. This is where they
converge, so the eye would see this point
of the eye over there. So it will actually see itself. And in general, these
type of diagrams are used to think about what
would the real or virtual image be of an object once the
light from that object either is reflected
off of a mirror, or goes through
some type of lens. And we'll be doing this a
lot more in future videos. But anyway, hopefully that
gets you a little bit familiar with the things we'll
be dealing with.