In this explainer, we will learn how to describe the image of an object that is formed by a plane mirror.
A plane mirror is the name given to a flat reflective surface.
This flat surface makes it easier to understand what will happen to light when it reflects off the mirror.
A more complicated mirror, such as a curved mirror, will reflect light differently. For that reason, only plane mirrors will be discussed in this explainer.
The picture below shows a plane mirror alongside a curved mirror.
The mirror on the left has a flat surface, so it is the plane mirror.
To describe what happens to light when it reflects off a plane mirror, we talk about images and objects.
An object, in this case, is something that is placed in front of the mirror.
Light coming off the object will reflect off the mirror, and an image of the object will be formed in the mirror.
The image below shows an object reflected in a mirror to form an image.
Notice that the image appears to be inside the mirror. This is not actually the case, it just appears that way.
A ray diagram will help explain what is seen in the mirror.
These ray diagrams show what is happening to the light when the object is reflected.
They use a side-on view of the object and mirror. Below is the ray diagram for this set up.
This ray diagram was constructed by first starting with light rays going from the object to the mirror.
This assumes that light is being emitted by the object and not some other source, like a light bulb.
This is done to simplify the problem, as we would have to draw more rays coming from the source.
Since the object is a source of rays, we can draw the rays starting from any point on the object.
However, that is not necessary, and to make the ray diagram as simple as possible, only two rays are used, a ray from the top and a ray from the bottom of the object. This is shown in the diagram below.
The rays from the object to the mirror, like all rays in the diagram, travel in a straight line.
At this point the two rays will both reflect off the mirror. The light ray is now traveling from the mirror to the person’s eye.
Again, this is shown in the diagram below.
Notice that the angle between the mirror and the ray from the object is the same as the angle between the mirror and the reflected ray.
This is always the case when light reflects off a mirror.
The observer now can see the light rays that reflected off the mirror.
However, the brain assumes that light travels in an uninterrupted straight line.
The dotted lines represent the apparent path of light rays from the image.
This is what forms the image and why it appears inside the mirror.
Example 1: Knowing What Is Meant by a Plane Mirror
Which of the mirrors shown is a plane mirror?
Answer
Remember that a plane mirror is a flat reflective surface.
Mirror B is curved.
This tells that the answer is mirror A.
Any similarities or differences between the object and the image will tell us about the way light is reflected by the mirror.
First, we will discuss a similarity. Any object reflected in a mirror will create an image that will appear to be the same size as the object.
More simply, the image will be the same size as the object.
The diagram below shows three objects of different sizes reflected in a mirror.
The largest object, which is the red one, has an image that is the same size. This is true for the green object and the yellow object.
Looking at just one of the cubes, a ray diagram shows how the light rays create an image that is the same size as the object. This is shown below.
The two distances shown clearly show that the height of both the image and the object is the same.
The order of object size is the same for both the objects and the images. The red object is the largest, followed by the green object, followed by the yellow one.
What is also clear from the image below is that the distance from the mirror to the object is the same as the distance from the mirror to the image.
The red object is the farthest object from the mirror, so the red image is the farthest image from the mirror.
The yellow object is the closest to the mirror, so the yellow image is the closest image to the mirror.
The picture below shows just the green cube.
The green object is located between the red and yellow objects. This is also true for the green image, which is located between the red and yellow images.
The order of object distance is the same for both the objects and the images. The red object is the farthest away, followed by the green object, followed by the yellow one.
This tells us that the distance an object is from the mirror is the same as how far behind the mirror the image appears to be.
This can be made clearer using a ray diagram.
More simply, the object-to-mirror distance will always be the same as the image-to-mirror distance.
Example 2: Knowing That Plane Mirrors Produce an Image of Equal Size to That of the Object
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?
- The image is larger than the object.
- The image is the same size as the object.
- The image is smaller than the object.
Answer
When a mirror reflects an object, it does not change its apparent size when we see its image.
The reason this is true can be demonstrated with a ray diagram.
Remember that we can choose any point as a source of the light rays, but the top and bottom of the object are chosen for this purpose.
This allows us to work out where the top and bottom of the image will appear to be.
Light travels in straight lines.
The light rays from the object to the mirror will reflect at an equal angle away from the mirror and to the person.
The dashed line represents the apparent path of light rays from image.
The correct answer is then B: the image is the same size as the object.
Example 3: Knowing That the Image Appears to Be the Same Distance behind the Mirror as the Object Is in front of the Mirror
An object is located 1 metre 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?
- Less than 1 metre
- 1 metre
- More than 1 metre
Answer
The object-to-mirror distance will always be equal to the apparent mirror-to-image distance.
The reason this is true can be demonstrated with a ray diagram.
Remember that we can choose any point as a source of the light rays, but the top and bottom of the object are chosen for this purpose.
This allows us to work out where the top and bottom of the image will appear to be.
Light travels in straight lines.
The light rays from the object to the mirror will reflect at an equal angle away from the mirror and to the person.
The dashed line represents the apparent path of light rays from the image.
This tells us that since the object is located 1 metre in front of the plane mirror, the image must appear to be the same distance behind the mirror.
The correct answer is B: 1 metre.
There is a difference between objects and images.
Using a mirror to reflect an image of an object is not the same as rotating the object.
However, we need to be careful when talking about the image being the reverse of the object.
The image below shows a correct object–image pair.
In the diagram above, we can see that the object, going from point A to B, has a blue half and then a red half.
However, going from A to B, the image has a red half and then the blue half.
The colors of the image appear to be the reverse when compared to the object.
What is happening may be made clear with the aid of a diagram from a side-on view. This is shown below.
Looking at both of the diagrams above, we can understand what is happening.
Remember that a mirror will not change either the object-to-mirror distance or the image-to-mirror distance.
So, since the red half of the object is closest to the mirror, the red half of the image must also be closest to the mirror.
Equally, the blue half of the image must be the farthest from the mirror as the blue half of the object is the farthest away from the mirror.
This is why the colors appear reversed.
We have chosen an object with a noticeable difference between it and its image.
For example, the colored cubes we previously looked at were reversed by the mirror, but we cannot tell because they are symmetric.
Letters have very specific shapes, so the way they are reflected is important.
The diagram below shows a word being reflected in a mirror. We may expect that the letters would be reversed, just like the colors in the cube above.
This is clearly not the case.
We can use the fact that a mirror will not change distances to explain this. Each letter of the word is at the same distance from the mirror.
If visibly different parts of an object are at different distances from the mirror, then the mirror will appear to reverse those different parts.
Since every part of the word is the same distance from the mirror, when it is reflected and an image is formed, there will be no apparent reversal.
If we change the object so that parts of it are at different distances from the mirror, we will see an apparent reversal. This is shown in the diagram below.
This diagram above shows how letters can be reversed by a mirror. Parts of the letter P are closer to the mirror than others.
This is why the letter is reversed.
Once again, it is helpful to show this using a ray diagram, as shown below.
The red ray comes from part of the letter that is closer to the mirror than the top and bottom of the letter.
This is why the letter appears to be reversed.
With this in mind, if we were to place the word so that parts of it were at different distances from the mirror, we would get a reversed image. This is shown in the diagram below.
Example 4: Understanding How a Plane Mirror Reverses an Object
Light from an object is reflected by a plane mirror. The shaded side of the object is marked with a symbol. The mirror produces an image of the object, as shown in the following figure.
Which of the following figures correctly shows the object if it is rotated so that the shaded side of it faces away from the mirror? The base of the object remains in contact with the surface that it initially rests on while the object is rotated.
Answer
This question is asking about the object and not the image.
The correct answer is option B as it is what the object looks like if it is rotated so that the shaded side of it faces away from the mirror.
Notice that both option B and the image have the shade on the same side, but the symbol is not reversed in the image.
The reversal of the symbol in this scenario has nothing to do with the object-to-mirror distance nor the image-to-mirror distance.
When we are rotating the object, the reversal of the symbol occurs only because we are rotating it.
This can be seen in the image below, which shows the rotation of the object in four stages.
The smaller arrows represent the direction of the rotation of the object.
The diagram below shows an incorrect object–image pair. It also shows the correct image.
Again we can use the fact that a mirror will not change distances to explain this.
Both the red and the blue halves of the object are equally distant from the mirror.
This means the same is true for the image and the colors do not appear to reverse.
Remember that if part of an object closer to the mirror is different to part of an object farther away, then the mirror will appear to reverse these different parts.
In this image, the different parts of the object are a red half and a blue half.
Since the different parts of the object are equally distant from the mirror, no apparent reversal takes place.
Of course, just like the cubes before, the image is being reversed, but it is not noticeable since there are no colors reversing.
Let us summarize what we have learned in this explainer.
Key Points
- A plane mirror is a flat reflective surface.
- An object and its reflected image appear the same size.
- The image appears to be the same distance behind the mirror as the object is in front of the mirror.
- A ray diagram allows us to trace the path of light that forms the mirror image.
- A light ray travels in a straight line.
- If visibly different parts of an object are at different distances from the mirror, then the mirror will appear to reverse those different parts.
