Ray Diagrams - Concave and Convex Mirrors - Light - CBSE Class 7 Science Video Lecture

Okay, in today's video I'm going to go over how to draw
ray diagrams for concave lenses and
convex mirrors.
This is the general diagram we're going to use. We need to know some important parts of this
diagram. One of the important parts of the diagram is the principal axis; it runs right through
the centre
of the lens in the horizontal direction
like the horizontal orientation.
Then we have F and we have 2F.
F is a distance from the lens that is
called the focal length.
2F is twice as far away so we call it 2F:
it's two focal lengths
away from the lens. Alright now, by the
end of the video
we want to be able to - wherever we
locate
the object that green arrow is the
object - we want to be able to say where
the image
is going to be, regardless of
where we put
the object, whether we put it at F, inside,
between, beyond 2F
for the image we want to be able to say -
first we want to be able to draw
the ray diagram, of course, and then for the
image
and we want to be able to say what the
location of the image would be,
the size, the orientation, and the type.
And really,
before you even draw the ray diagrams
for this type of lens and the other
types of lenses
or mirrors you should really know what
your answer is going to be
so that you can draw them properly and
just know what's going on ahead of time.
Alright, so this is our first diagram
and you can see we have the concave
lens
and the object is between F and 2F.
Now, for all of these - both lenses and both
mirrors, concave and convex,
I told you that these are all drawn the
same. Now, they're not drawn exactly the
same but they're drawn basically the same.
So, for example, the first ray I like to
draw is the ray that goes into the lens parallel.
And then this is a concave lens, it's a
diverging lens so parallel rays
are going to diverge. Okay, now the
question is how much is it going to
diverge? How do we know how much to bend this angle
when the light is refracted, when it passes
through that lens?
In the past, I've told you we draw this
first ray so that it goes parallel
in and comes out through F. Now, we're not
going bend it down this way through F,
we're going to bend up this way and we
bend it up that way so that it appears to
come from
F on the same side of the lens
as the object. So just like we said
before
this is called parallel F. the diverging
lens.
it diverges so that looks like the ray
is coming from F on the same side as the
object.
So that's the first one: parallel F.
The other one, we said, is
F parallel so we draw this one as
enters the lens as if it was going
to F on the other side. But when it
comes out,
it goes parallel. So that is the
second ray
and your eye is on this side and it
follows that ray backwards
and you trace it backwards with carefully with your pencil and your ruler
and a nice sharp pencil
and it follows it back in that direction. Okay, so
those are the first two rays we always draw,
the first one being parallel F, and F
parallel. This is for a diverging lens
obviously.
The third one we draw goes right through the center and we drew
the same thing for the convex lens.
It goes right through the center and it
doesn't
refract, it doesn't bend, it goes right through the center like that.
you'll notice that your eye is over here, it
sees those three light rays coming
out of the lens as they're reflected off of
the object,
it follows this one back, comes back through F,
this one follows straight back,
this one goes
F parallel: your eye follows it straight
back.
Your eye follows this one straight back, and your eye follows this one straight back,
and the image appears right there. That
is our image.
You should remember that in the
previous
two videos, the one for the convex lens
and the concave mirror, there were five
different ray diagrams you needed to know
how to draw.
For the concave lens, there's only
one.
It doesn't matter where the object is
on this side of the lens, the image is
always going to be between
F and the lens and I say behind the lens -
on the same side of the object you can
always say - but I say behind the lens because
here we are with our eye,
it's behind the lens on
the same side
lens as the object. It's always going to
be
between the lens and F.
It's always going to be right side up, it
doesn't matter where we put the object,
it's always going to be right side up. It
doesn't matter where we put the object,
the image is always gonna be smaller than the object.
Of course, unless you bring the object
right up to the lens,
then it gets to be the same size, but for
the most part
it's always gonna be smaller and it's
always going to be a virtual image.
This is an image, this is an image that
is created by
diverging light rays. Okay, so once again
there's only one case it doesn't matter
where we put the object
the image is always going to be between
F and the len, it's always going to be right
side up, it's always gonna be smaller, and
it's always gonna be virtual. The
image will never appear
on this side and will never appear
beyond F on this side.
alright let's go on and do
the convex mirror. So, once again we
have these
important places we have to know about:
principal axis
the center of curvature, and the focal point.
And this is the focal
length, and the focal length
is one-half the center of curvature. Now, this is a convex mirror, it's a diverging mirror.
This side, the left side of the mirror, is
the shiny side.
You'll notice the
focal point and the center of
curvature
are on the other side - or as I like to say,
behind the mirror. Okay, this is a mirror that's curved outwards,
you're over here, the object is over here
the focal point and the center of curvature
are on
the back side of that mirror. Just
think of it - it's a spherical mirror, it's a big
circle,
C is the center, F is the focal point,
it's halfway between the center: which,
this distance from the center of
curvature to the mirror is the radius,
and the focal point is halfway between
the center of curvature
and the mirror. Okay, so once again
regardless of where we put the object (the green arrow being the object)
you need to be able to draw the ray
diagrams to locate the image,
and then you should be able to say where
the image is going to be located,
what size it is going to be, what orientation it is going to be, and what type it is going to be
And really, once again you should be
able to say all those things
even before you draw the ray diagram,
you should really know how this
mirror works
before and that will help you to draw the
ray diagram properly.
Here is that diagram we're going
to use,
here's your eye, here's the object, here's
the mirror.
once again the left hand side , the side that's bending out,
the convex side, is the shiny side and
we're going to draw these just like
we've drawn all the others:
parallel F, F parallel so we draw the
first one.
The first ray goes into the mirror
parallel
and then it's going to diverge, it's
going to be reflected.
Now how do we know how big to make this
angle or how do we know how it's
reflected?
Well , it has to be reflected as the if it
was coming from
F, and your eye is over here and sees this
light ray coming
out of the mirror, it does not know that it was reflected right here so it assumes it
traveled in straight line
so it assumes it came from F. So, this
light ray is bent,
is reflected in such a way that it
appears to come from F.
Of course it does not because this is a
solid mirror and light can't travel through the mirror.
So, that's our first ray just like we
did in all the others,
parallel F. So the next one of course is
going to be
F parallel, it's going to go
into the mirorr as if it was going towards F,
but when it comes off of the mirror, it comes out parallel to the principle axis.
Your eye sees that light ray coming out
of the mirror,
it assumes it traveled a straight line, it doesn't know it was reflected off of that that
mirror and therefore goes back in your eye,
follows that
light ray back behind the mirror.
Does it actually come from behind
there? No. But your eye, your brain
thinks that it does. The next one
that we draw
the third one is kinda just to check.
It's going right toward C so I draw it
with my ruler
in my nice sharp pencil right towards C,
of course if it's going toward C which
is the center of curvature.
It's already perpendicular to this mirror
right here, so it's reflected straight
back.
Your eye sees that light ray reflected
straight back
and assumes it came in a straight line
and assumes that that light ray came
from the center of curvature.
Okay, once again you'll notice that they
all converge right here and that is
where the image appears.
The image appears right there on the
other side behind
the mirror. Okay now
once again just like we did in the
previous for the concave
lens, for the concave mirror there's only
one case we have to draw
really because regardless of where we put
the object
the image is always
going to be between F and the mirror.
Remember the previous one was between F
and the lens, this is between F in the
mirror and it's behind the lens
Excuse me, this should actually say "behind the mirror"
it's going to be behind the mirror, this is
behind the mirror.
This is your eye, this is the other
side, I call this behind.
Okay, it's between F and the mirror, behind the mirror.
It's always going to be right side up,
it's always going to be smaller than the
object
unless you bring the
object right up to the mirror
and it's always going to be virtual,
it is an image that is created by
diverging light rays. Okay, so once again
I just want to point out there's only
one case,
remember, for the convex
lens and the concave mirror. There
were five different cases you need to
know
but for the concave lens and the convex
mirror
there's only one case because regardless of where we put the object
the image will always be between F
and the mirror, it'll always be erect
it'll always be smaller and will always
be a virtual image.
Thanks very much for
watching
I hope that was helpful and we will see
you in the next video.
Thank you very much!