# Mirror

Have you ever wondered how a mirror produces an exact image of you when you stand in front of it? Is it some sort of witchcraft and sorcery? Or can we explain this by scientific reason? There is no need to resort to magic. As with most of the phenomena we observe on our planet, science can explain it. But how do mirrors work? How many different types of mirrors are there and what are their properties? And how can we use them to look at planets that are light-years from us? At the end of this article, you will be able to answer all of the above questions and more. The next time you find yourself admiring your beauty in front of one, you can also admire the physics behind it!

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## Mirrors Physics Definition

In physics, a mirror is a smooth surface that reflects a great amount of the Light falling on it.

When Light reaches a surface, part of it is reflected, and part of it is absorbed. Mirrors are excellent reflectors. Nearly all the Light that is incident on the mirror bounces back, and just a small portion of it is absorbed.

A virtual image forms when light rays appear to come from a source that is not really there.

Reflections can be categorised into two types based on the surface on which light is reflected. When light falls on a smooth, shiny surface, Reflection happens in a straight line and single direction, this is called specular Reflection. But if when light falls on a rough and uneven surface, the reflection is in multiple directions, this is called diffuse reflection.

Fig. 1: When the reflection surface is smooth, light reflects in a single direction but when the surface is rough or is a collection of small particles light is reflected in multiple directions.

A mirror is often produced by adding a thin layer of reflective material such as aluminum or tin to a piece of transparent glass.

## Types of Mirrors in Physics

Mirrors can be classified based on curvature. If the curvature radius is zero, it's called a plane mirror. These are mirrors that you see most commonly used in homes and salons. In a plane mirror, the proportions of the image and the object are always the same.

However, mirrors can also be curved! This category is called spherical mirrors, which are again divided into concave and convex mirrors. The specific details are not important for your GCSEs, but you can read about their applications below. It's interesting as they have very common applications. The next time you see them somewhere, make sure to check them out and how they help us in seeing the world around us?

A mirror that bulges inward is called a concave mirror. An easy way to remember this is to imagine a cave where the surface inside is a mirror hence the name conCAVE. See how they're used in our daily lives.

Fig. 2: Diagram of a concave mirror.

Concave mirrors are used inside flashlights as reflectors to concentrate all the light in a single direction. These reflected rays have a high intensity and can travel long distances before fading away.

Fig. 3: Concave mirrors used for magnification in dentistry,

Concave mirrors are also used in dental mirrors as they produce an enlarged image inside the mouth. Wait! There's another spherical mirror that's the opposite of the concave mirror. A mirror that bulges outwards is called a convex mirror. Imagine a hemisphere where the outer surface is a mirror.

Fig. 4: Diagram of a convex mirror.

These mirrors allow you to see at a wider range than you can with a plane mirror. So can you think of places where they can be used?

Fig. 5: Example of a convex mirror.

You might have noticed large convex mirrors used in department stores. These convex mirrors have a greater field of vision than a plane and concave mirror, producing an image covering a larger area. They're placed in blind corners to help people not run into each other by providing a view of the other side.

Fig. 6: The convex mirror is used in side-view mirrors of a vehicle.

They're also used in side-view mirrors in vehicles. You might have observed the warning "Objects in the mirror might appear closer than they are". Convex mirrors are used in automobiles because they provide an upright image and a wider field of view due to their curved outwards design. Due to this property, the distance of the image will appear to be smaller than in reality.

## Mirrors Physics Simulation

We can represent a plane mirror by drawing Ray Diagrams. These diagrams help us to predict the type of image that a mirror forms. There are two types of images: real images and virtual images. Let's see the definitions for these types of images and some examples of image formation by mirrors using Ray Diagrams.

A real image is formed when the reflected rays intersect in front of the mirror.

A real image can be projected onto a screen. You won't see this property in-plane mirrors. But it's possible in concave mirrors.

The light of an object that reflects on a concave mirror produces a real image. Since the image is real, we can project it on a paper sheet by placing it where the image forms.

Fig. 7: Lightrays reflected from a spherical concave mirror form a real image that can be projected on a screen.

As shown above, in a ray diagram, each ray of light is depicted by a straight line, with an arrow to represent its direction.

A virtual image is formed when the reflected rays intersect behind the mirror.

Such an image cannot be projected onto a screen. Plane and convex mirrors always produce virtual images.

Fig. 8: The image formed by a plane mirror will always be virtual, laterally inverted and upright

We can also use ray diagrams to determine the image's position and its size. Consider an object reflecting from a plane mirror, like in the above image. We can draw two light rays coming from the object. This is a common practice that you will see in most ray diagrams. The point at which these rays intersect indicates the position of the image. This is not possible if we use a single ray of light!

The incident rays coming from the object bounce off the mirror, and we see the reflection of the object or image as these reflected rays enter our eyes. To obtain the position of the image, all we need to do is extend the reflected rays behind the mirror, and see where they intersect. You will notice that the image is formed at the same distance behind the mirror at which the object is placed in from of the mirror. The object will be laterally inverted, and the image will be of the same size.

## The Formula of a mirror in physics

Let's see a diagram of a plane mirror reflecting light.

Fig. 9: Ray diagram of a plane mirror with labels

There are many new terms that are useful to describe the reflection of light. Let's have a look at each of them.

The normal is a line that passes through the centre of a mirror.

The normal is used as a plane of reference while pacing the object. The distance between the object and image is measured on this axis.

The angle between the incident light ray and the normal to the mirror is called the angle of incidence.

The angle between the reflected light ray and the normal to the mirror is called the angle of reflection.

Now that we have a better understanding of the lingo, we can talk about the special relationship between the angle of incidence and reflection. This relationship is described by the Law of Reflection.

The Law of Reflection states that the angle of incidence is equal to the angle of reflection.

This can be mathematically represented as follows

$\theta_r=\theta_i$

or in words,

$\text{Angle of reflection}=\text{Angle of incidence}$

Remember that these angles are measured with the normal at the point where the rays meet the mirror and not with the mirror itself. Look at the image below for reference.

## Importance of Mirrors

To highlight the importance of mirrors, let's now consider a particular array of them that is extremely useful: an L-shaped mirror. Below is a diagram for an L-shaped mirror:

Fig. 10: The ray diagram for an L-shaped mirror shows how a ray is reflected back, parallel to the incident ray

Notice that there are two locations where reflection occurs. This causes the ray of light from the source to be reflected back in the same direction the ray had when it was incoming. This phenomenon is called retroreflection, and it works regardless of the angle of incidence. Moreover, we can scale up this setup by adding a third mirror perpendicular to these two. The result is a mirror with the shape of the corner of a cube. In this three-dimensional array, any light ray is always reflected directly back to its source.

Did you know that in 1969 as part of the Apollo 11 moon mission, astronauts were asked to leave a panel with retroreflectors? This was done so that we could aim a laser from the earth at them and guarantee that light would bounce exactly back in the same direction. Some of the most precise measurements of the distance between the moon and the earth we have to this day were obtained with this method.

Fig. 11: A panel with retroreflectors was deployed to reflect a laser beam back to its source on earth.

Another important application of mirrors is the reflecting telescope. These telescopes consist of an array of mirrors that work together to focus and magnify an image. In a reflecting telescope, the light reaches a mirror located at the bottom of the telescope. This is a spherical mirror that focuses the light at the top end. Then, it is reflected by a second mirror to a position outside the telescope, where the observer can see the distant object's image enlarged.

Fig. 12: Reflecting telescopes use an array of mirrors to focus and magnify the image.

## Mirrors - Key takeaways

• In physics, a mirror is a smooth surface that reflects a great amount of the light falling on it.
• Mirrors can be classified as plane and spherical.
• In a ray diagram, the normal is a line that passes through the centre of a mirror.
• The angle between the incident light ray and the normal to the mirror is called the angle of incidence.
• The angle between the reflected light ray and the normal to the mirror is called the angle of reflection.
• The law of reflection states that the angle of incidence is equal to the angle of reflection.
• The plane mirror can be represented by drawing ray diagrams. They allow us to predict the type of image that is produced i.e. whether the image is real or virtual.
• A real image is formed when the reflected rays intersect in from of the mirror. A real image can be projected onto a screen.
• A virtual image is formed when the reflected rays intersect behind the mirror.
• A plane mirror will always produce a virtual, laterally inverted, and upright image of the object in from of it.

## References

1. Fig. 1: Specular v Diffuse reflection (https://commons.m.wikimedia.org/wiki/File:Specular_v_Diffuse_reflection.png) by Dr. Dalia K Maraoulaite (https://commons.m.wikimedia.org/w/index.php?title=User:Dr_Maraoulaite) is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/).

#### Flashcards in Mirror 13

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What are mirrors?

A mirror can be defined as a reflective surface that reflects all the light falling on it. This property produces an image that we see as a reflection.

What is the formula of mirrors in physics?

The mirror formula is as follows:

1/u + 1/v = 1/f

• u = Distance of the object from the mirror
• v = Distance of the image formed from the mirror
• f  = focal length of the mirror

What are the types of mirrors in physics?

MIrrors can be classified into a plane and spherical mirrors. Spherical mirrors can be further classified into concave and convex mirrors.

What is mirror reflection in physics?

The reflection is when the light rays bounce back when interacting with the surface of a mirror.

What is importance of mirror in physics?

Mirrors have several applications. For example, the L-mirror and the reflecting telescope.

## Test your knowledge with multiple choice flashcards

When light rays fall on a mirror it is ...

Which surface would provide the best reflection?

An image produced by a plane mirror is ...

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