Mirror

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?

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

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

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

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.

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.

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.