Reflection at Plane Surfaces: Definition, Image Formation, Diagrams

Reflection at Plane Surfaces: When a ray of light is incident on a plane surface, it bounces off the object. This phenomenon is called reflection. Basically, due to reflection, we can look at the object and get to know its colour. When white light falls on an object, it absorbs all the colours in it and only reflects back some colours. That reflected colour becomes the colour of the object. So, reflection plays an important role in our life to see this colourful world. 

In this article, we shall see in detail the reflection of light from the plane surface and image formation through it.

Reflection of Light

When a ray of light is incident on a surface, it bounces back from that object. This phenomenon is called reflection. Depending on the shape and structure of the surface, it reflects back accordingly. Consider the diagram shown for a better understanding below:

Here, two different rays of light are shown with two different colours. Both orange and black light are incident at different angles on the uneven surface. Since the angle of incidence of both the rays of light is different, the angle of reflection is also different. Hence, shape and structure play an important role in understanding the reflection of light from a surface.

Laws of Reflection

Consider the diagram below to understand the laws of reflection:

1. The angle of incidence is equal to the angle of reflection when light is reflected on a plane surface.

2. The incident ray reflected the ray and the normal lie on the same plane.

Image Formation Through a Plane Mirror

For reflection through a plane mirror, we will assume that the mirror is completely smooth and is not at all rough. Consider the diagram below, which shows the image formation due to a plane mirror

An object of height \(AB\) is placed in front of a plane mirror \(MM’\) at a distance \(u\) from it. There is a possibility of many different rays of light that gets reflected from the surface. Some of those rays are shown in the diagram. 

The ray \(AP\) is incident normally on the mirror. Since it is incident normally on the surface, the angle of incidence is \({0^ \circ }.\) According to the laws of reflection, the angle of reflection is equal to the angle of incidence; hence the angle of reflection is also \({0^ \circ }.\) This means the ray will retrace its original path. Another ray \(AO\) is an incident at some random angle on the plane mirror. Then the ray \(OC\) is reflected at the same angle with respect to the normal as that of \(AO.\) The same process goes for the other rays too. 

When all the rays are extended back, all of their incidents are at the same point where image formation takes place. When traced back, the image formed \(A’B’\) has the same height as that of the object and is at the same distance as that of the object. Hence, by studying the reflection, we can see that:

\(u=v\)

Which says that object distance is equal to the image distance for the plane mirror and, 

\(h’ = h\)

Which says that the height of the image is equal to the height of the object. If both the height is the same, the image is not magnified, which means magnification through a plane mirror is always \(+1.\)

Image Formation through Two Plane Mirrors

First, we will consider two plane mirrors \({M_1}\) and \({M_2}\) which are placed parallel to each other. An object is placed somewhere between both the mirrors. We will consider reflection through one plane mirror at a time.

Let us say that the ray of light from the object is the first incident to the mirror \({M_1}.\) The image \({I_1}\) is the first image formed directly through the reflection through the object? The ray of light gets reflected and then is incident on the second mirror \({M_2}.\) This directly means that the image formed due to reflection after the first mirror \({I_1}\) will act as an object for the second mirror. Hence, the image formed after the second reflection will be \({I_{12}}.\)

Now the reflected ray from the mirror \({M_2}\) (ray number \(3\)) will be an incident on the mirror \({M_2}.\) That means the image \({I_{12}}\) will act as an object for that mirror. The image thus formed will be an image \({I_{121}}.\) Now again, the reflected ray will be incident on the parallel mirror, and again, the same phenomenon will get repeated. So, there are infinite possible rays that continuously gets reflected in between both the mirrors. Hence, the number of images due to two parallel plane mirrors are infinite.

For the next case, we will consider two plane mirrors which are placed such that they make an angle \({90^ \circ }\) with each other. An object is placed in between at a random angle between both the objects. The incident ray \(OB\) on the mirror \(MM\) forms an image \({O_1}\) behind the object, and the reflected ray from \(MM\) gets incident on the mirror \(MM’\) and image \({O_1}\) acts as an object for \(MM’.\) The image thus formed due to reflection from \(MM’\) is termed as \({O_3}.\) Now, after getting reflected from \(MM’,\) there are no possible rays that are incident on \(MM.\)

If we repeat the same process for the reflection from mirror \(MM’\) first, we get the image \({O_2}\) which acts as an object for \(MM\) and the image thus formed will be \({O_4}.\) This image will be at the same place as \({O_{3,}}\) which means both the images overlap each other and form a single image. Hence, the total number of images formed in this case is \(3.\)

Similarly, if we repeat the same procedure for two mirrors with different angles between them, we get a different number of images. Lesser the angle, more are the number of images, and more is the angle, less is the number of images formed. And with more practise, we can also study image formation through two or more mirrors.

Summary

From this article, we got an introduction about reflection and why is it important for our daily lives. It is considered to be impossible to see an object or understand its colour without reflection. Based on reflection only, we can see different coloured objects. Apart from this, we studied the laws of reflection, where we have two laws. One of them states that incident angle and reflected angle must always be the same, and the other one says that incident ray, reflected ray and normal are situated in the same plane. 

Then we studied the image formation through a plane mirror. After looking at it thoroughly, we came to know that image is formed behind the mirror at the same distance as the object and of the same height. And finally, we saw how reflection takes place through two mirrors at different angles and came to know about the number of images formed after reflection. We studied the lesser the angle; the more are the number of images and vice versa.

Frequently Asked Questions (FAQs)

Q.1. What is a reflection from a plane surface?
Ans: Reflection from a plane surface indicates the understanding of how image formation takes place through a plane surface or how we can see an object through-plane surfaces. The colours reflected on the surface after white light is incident on it is the colour of the object.

Q.2. What are the properties of reflection from the plane surface?
Ans: The laws of reflection are\( – \left( 1 \right)\) incident angle is equal to the angle of reflection and \(\left( 2 \right)\) incident ray, reflected ray and normal lie in the same plane.

Q.3. How is a reflection in-plane or smooth surface?
Ans: Reflection from a plane or smooth surface is regular, which means all the incident rays are parallel to each other, and all the reflected rays are also parallel to each other.

Q.4. What kind of reflection occurs on a plane mirror?
Ans: The kind of reflection that occurs on a plane mirror is regular reflection.

Q.5. What is the type of image formed after reflection from a plane mirror?
Ans: The image formed after reflection from a plane mirror is virtual, upright and the same as the size of the object.

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