In this video,
my aim is to give you the basic understanding of the terms
virtual and real images.
It’s a slightly long video, but it’ll be one of the most interesting ones too!
I’m sure you’ll enjoy it!
What you see on your screen is a mirror.
It has this plane and smooth reflecting surface on one side.
Its called a plane mirror
because its reflecting surface is flat.
What other types of mirrors have you seen?
Have you seen this type of mirror?
Well, you must have seen it on the stand of your bathroom or maybe
hung up there on a wall.
This is not a plane mirror.
Try to move your hands over the surface
and you will notice it.
This is called spherical mirror
or more precisely spherical concave mirror.
It is curved inwards.
It is kept there as we use it mostly when we are shaving.
Now the obvious question that must be coming to your mind is,
why do we use concave mirror for shaving instead of a plane mirror?
For the answer to this question,
you have to wait for our future videos!
For now, we will be concentrating on the plane mirror only.
So let’s consider this plane mirror again.
When we stand in front of a mirror,
we see an image of ourselves in the mirror.
Nothing surprising there!
Why are we able to see our image?
Yes, it's due to the reflection of light!
In fact, it is due to multiple reflections of light rays.
The light rays from the source are first incident on our body.
They bounce off from our body and fall on the surface of the mirror,
and again gets reflected from there.
This reflected light then reaches our eyes and we see our image!
So what does this tell us?
This tells us that a light ray can get reflected multiple times!
The same thing happens for every other non-luminous object as well.
If you remember,
my initial goal was to explain what virtual and real images are.
Let me ask you a question!
This image that you see here,
is it a real image or a virtual one?
This image is a virtual image.
In order to explain what a virtual image is,
let’s first simplify this image.
Let’s represent this boy with an arrow pointing up.
And here is our plane mirror.
You can assume the top of the arrow
to be the top of the head of a boy.
Now where is the image of the boy formed?
When we see the image in the mirror,
it seems to us that the image is formed somewhere behind the mirror.
But there is nothing BEHIND the mirror right?
For us, the light rays seem to come from the location
present behind the mirror.
consider a few light rays emerging from top of the arrow.
All these rays are incident on the mirror and get reflected back.
All the reflected rays
will follow the laws of reflection!
You see that all the reflected rays are moving away from each other,
they are DIVERGING.
But we know that the image is formed somewhere behind the mirror!
So let me extend these reflected rays backwards and see what happens.
We see that they meet at this point BEHIND the mirror.
And to us,
this is where the top of the image appears to be.
When we look at this point
it appears to us that all the light rays are diverging from this point.
But in reality,
there are no light rays behind the mirror.
This being a smooth and shiny surface
reflects most of the light back,
and almost no light reaches behind.
we have the perception that the light rays are coming from the point behind!
So we just
considered the top of the arrow here.
We can do something similar for the remaining
points on the object to get the complete image!
So we say that the image formed here
is virtual or in other words not real.
You can think of a virtual image as an image
which appears to be formed in a position somewhere behind the mirror.
But in reality, it isn’t there.
A virtual image is formed
when the light rays diverge after reflection from the mirror.
Every plane mirror forms a virtual image.
This is the first characteristic of the images formed by plane mirrors.
Images formed by plane mirrors are VIRTUAL and not real.
So what is the real image then
and how are they formed?
Let’s replace this plane mirror
with a spherical concave mirror.
It’s because, real images can be formed by concave mirrors.
Let’s consider the light rays from the same point
of the object incident on the mirror.
We see that after reflection
each ray is travelling in a certain direction.
Note that here as well,
each ray obeys the law of reflection.
What difference did you notice here
compared to the plane mirror reflection?
In the previous case,
all the reflected rays were diverging.
here all the reflected rays after
reflection are INTERSECTING each other at this point or
we can say that they’re CONVERGING at this point.
And the image of top of the arrow
is formed at this point.
And we can do something similar
for the remaining points of the object too.
We get this image!
This is called a REAL image.
It is the image formed
when light rays CONVERGE after reflection from the surface.
One important thing we need to know
is that real images formed by
concave mirrors are always inverted!
We will learn more about this in the future videos!
You see that in this case,
the image is formed on the same side of the mirror as the object is.
Real images are always formed
on the same side as that of the object.
And light rays do reach this position.
When we look at the real image formed by a concave mirror,
it also seems to us that the light rays are diverging.
But here the rays actually are diverging from a point in front of a mirror.
Whereas in the case of a plane mirror,
it appears to us that the rays
were diverging from the point behind the mirror.
So did you understand the difference between
a virtual image and a real image?
When we look at a virtual image,
it appears to us that the light rays are
diverging from a point somewhere
behind the mirror.
But in reality they aren’t, as no light reaches behind the mirror.
It is only our perception.
And in the case of a real image,
the light rays are actually diverging from the point
present on the same side of the mirror.
light rays indeed reach this point.
Hence, we call it a real image!
I mentioned earlier
that concave mirrors form real images.
In fact they form both real and virtual images.
And what does it depend on?
It depends on the distance of the object from the mirror.
We will learn more about the real images
formed by concave mirrors in our future videos.
First, let’s go back to our plane mirrors.
This is only one of the characteristics of
an image formed by a plane mirror.
We will see more in our next video.