Lesson Video: Mirrors Science

In this video, we will learn how to describe the image of an object that is formed by a plane mirror.

09:39

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.

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