Video Transcript
In this video, we will learn how to
describe the image of an object that is formed in a plane mirror.
Before we talk about images,
though, let’s see what it means for a mirror to be a plane mirror. In this case, the word “plane”
means a flat surface. Of these two mirrors, this one is
flat, but this one we can see is not. Therefore, the mirror on the left
is not a plane mirror. The way to know for sure if a
mirror is plane is to look at it from the side. If the edge of a mirror is a
straight line like this one is, seen from the side, then the mirror is flat, and
therefore it’s a plane mirror.
A mirror can have any shape and
still be a plane mirror. The mirror could be, say, a circle
or a hexagon or have any shape. And so long as the mirror was flat
from the side view, it would be a plane mirror. Now, the reason that mirrors are
useful is they reflect light. If we put an object in front of a
mirror, then light coming from the object would bounce off the mirror and form what
is called an image. So here is the object. That’s the thing that exists in
real three-dimensional space. And here is the image of that
object, as seen in the mirror.
The image is actually not real. It’s a picture of something that
is, the object. To see how this works, imagine
we’re looking at our mirror and object from the side. So here is our plane mirror. And this is the object in front of
the mirror. In order to see an image of this
object in the mirror, there needs to be someone or something doing the seeing. Say that we place our eye here,
looking at the mirror in this direction. When we do, we’ll see an image of
this object. As we mentioned, this is because
light reflects off of the mirror. Light rays coming from all points
on the object bounce off the mirror and reach our eye.
Now, here’s something
interesting. When our eye sees these light rays,
it doesn’t know that they’re reflected off the mirror. As far as our eye can tell, these
rays have been coming in the same direction forever. Our eye traces these rays backward
and sees an image of the object. Now, here are two interesting
things about images in plane mirrors. Number one, these images have the
same size as the object. They’re not bigger or smaller or
different in shape in any way. Second, if we measure the distance
from the mirror to the object and the distance from the mirror to the image, we
would find those distances are the same. For a plane mirror, the images in
the mirror appear to be as far behind the mirror as the object is in front of
it.
Now, remember also that while the
object is real — it really exists — the image does not. If the plane mirror were hanging on
a wall, for example, we know that no image in the mirror can actually exist behind
the wall, which shows us that even though we can see the image in the mirror, it
doesn’t really exist like the object does.
So far, we’ve looked at a fairly
simple object being reflected in a mirror. Imagine, though, that we join a
green block to the back of this orange block we started with. Seen from the side then, our object
would now look like this. The image of our new object would
actually look like this. Notice that for our object, the
green block is in front and the orange block is behind. But then this is reversed in the
image. This happens because of the way
light reflects off the mirror. The part of the object that’s
closer to the mirror, the orange part, is reflected to appear as the front of the
image. Meanwhile, the part of the object
farthest away from the mirror, the green part, appears as the back of the image of
that object. This effect is known as front-back
reversal. It tells us how an object and the
image of that object in a plane mirror appear differently. Knowing all this about plane
mirrors, let’s look now at a few examples.
Which of the mirrors shown is a
plane mirror? (A) Mirror A, (B) mirror B.
A plane mirror is a mirror whose
surface is flat. There’s no bend or curve to the
surface of a plane mirror. We see that mirror B is curved and
therefore cannot be a plane mirror, but that mirror A is not. This mirror is flat. Of these two mirrors, mirror A is a
plane mirror.
Let’s look at another example.
Light from an object is reflected
by a plane mirror. The mirror produces an image of the
object. Which of the following correctly
describes how the sizes of the object and its image compare? (A) The image is larger than the
object. (B) The image is smaller than the
object. (C) The image is the same size as
the object.
To see which answer is correct,
imagine we look at a plane mirror from the side. Since the mirror is plane, from
this perspective, it will look like a straight line. If an object is put in front of the
mirror and an observer looks at the mirror, then that observer will see light from
the object that is bounced off the mirror. By tracing those reflected light
rays backward, the observer can see an image of the object. Our question is, is that image
larger, smaller, or the same size as the object? Since we have here a plane mirror,
that is, a flat mirror, it will always reflect light so that the size of the image
matches the size of the object. That is, the image and the object
have the same size. If our mirror was not a plane
mirror, it might reflect light so that the image is not the same size as the
object. But because it is plane, they are
the same size.
Let’s look now at another
example.
An object is located one meter in
front of a plane mirror. Light from the object is reflected
by the plane mirror that produces an image of the object. Which of the following correctly
describes how far behind the mirror the image appears to be? (A) Less than one meter, (B) one
meter, (C) more than one meter.
Let’s begin by drawing a sketch of
this scenario. Say that here is a plane mirror
seen from the side, and then here is an object. And the object is a distance of one
meter from the mirror. Light from the object is reflected
by the mirror. These reflected light rays, when
traced backward, form an image. It’s the image of the object. Because the mirror is a plane
mirror, the distance from the mirror to the image is the same as the distance from
the mirror to the object. In this case, that’s one meter. For a plane mirror, it’s always
true that the distance from the mirror to the object is equal to the distance from
the mirror to the image.
Let’s look now at one final
example.
Light from an object is reflected
by a plane mirror. The mirror produces an image of the
object. Which of the following figures
correctly shows the image and the object?
From our perspective, the object
looks like this, a red box in front of a blue box. These answer choices are different,
though, in what they show in the plane mirror, that is, the image of this object in
the mirror. In answer choice (A), the image has
the red box in front and the blue box behind. This is reversed in answer option
(B). Here, blue is in front and red
behind. So which option, (A) or (B),
correctly shows the image of this object? To figure this out, we can recall
that in plane mirrors, images experience what is called front-back reversal.
Consider a plane mirror looked at
from the side, so it looks just like a flat line. In that case, our object would look
like this, with the blue section being closer to the mirror than the red
section. Front-back reversal means that when
an image of this object is formed, it will appear reversed front to back from the
object. That is, for our object, the blue
section is on the right, but for the image, the blue section is on the left. This is called front-back reversal,
though, because we’re usually looking at an object like this so that what we call
the front of our object, in this case the red section, then becomes the back of our
image.
Notice that in answer option (A),
this front-back reversal doesn’t happen. In this choice, the front of our
object is red, and the front of our image is also red. Answer choice (A) then doesn’t
display this reversal that really does happen. For our answer, we choose option
(B). This figure correctly shows an
object and the image of that object in a plane mirror.
Let’s now finish this lesson by
summarizing a few key points. In this video, we learnt that a
plane mirror is a flat surface that reflects light. The image of an object in a plane
mirror is the same size as the object. Along with this, the distance of
the object from the mirror is the same as the distance of the image from the
mirror. And lastly, we saw that when we
have a three-dimensional object, the image of that object in a plane mirror is
reversed front to back. This happens because of how light
travels from the object and reflects off the mirror. This is a summary of mirrors.
