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Reflection


When we are standing before the mirror we are able to see our image and when we speak we could observe sometimes that those sound waves are heard after some time delay. Why are all these things observed? These are due to the phenomena of reflection. Reflection occurs both in light and sound.

 

What is Reflection?

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Reflection is a very common word and is related to the fields of physics and mathematics. The basic meaning of a reflection is ‘Something Returned in Response’. Even in our conversation we use this term at times, to mean a some one’s reaction. In optics, it relates to an image of an object.

Reflectional Symmetry

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                                                    Reflectional Symmetry
Look at the above diagram. An object A is placed in front of a mirror M. Consider rays AB and AO from A. As per law of reflection, the ray AB is reflected back as BA and ray AO reflected as OC.

Retracing these rays behind the mirror they appear to intersect at A'. This is where you find A', an image of A and it appears to be at the same distance behind as much as the object is before the mirror. That is the measures x and x' are same, though you cannot actually measure x'. The creation of image A' is actually called as the reflection of A.

Reflection of Light

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Light rays are basically wave motions and they are capable of being reflected by an interface in between two mediums. Such interfaces are called Light Reflectors

The most common Light reflector is the one we use many times a day, which is nothing but a plane mirror. But it is not only a plane mirror reflects light. Even a curved mirror does that though the nature of reflection of light by curved mirrors is different. Let us discuss this part separately at a later point.
Now for the present, let us concentrate on reflection of light by a plane mirror described by the following diagram.
                                                Reflection of Light

M is a plane mirror. It is a plane glass but on one side (the hatching show the surface) a suitable abrasive is coated to give the reflection effect to the glass, that is to make the glass to be a plane mirror. It also acts as an interface to separate the mediums on the left and right to that.
Let O be a any point on the plane mirror. The line NO is the perpendicular or better called as ‘principal’ to the plane mirror at O. Suppose a light ray AO hits the plane mirror at O, it is ‘reflected to the same medium as ray OA’. This is called Reflection of light.
Reflection of light obeys certain condition. The condition is called either as ‘Law of Reflection’
As per the Law of Reflection:
  1. The angle of incidence is always equal to the angle of reflection.
  2. The incident ray, the reflected ray and the normal all lie in the same plane.
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Refer to the diagram shown above,here NO is the principal axis. The angle AON made by the ray AO with the principal axis is called as ‘Angle of Incidence’. The Angle of Incidence is the angle made by the ray incident on the surface from the line which is normal to the plane at the point of incidence.
The angle of incidence is very important as that decides the nature of the reflected ray.
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Angle of Reflection

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Referring to the same diagram,the angle A'ON between the reflected ray OA' and the principal axis NO is called as ‘Angle of Reflection’.
The Angle of reflection is the angular reflection formed by a reflected ray and a perpendicular to the surface at the point of reflection.The Angle of Reflection depends on the Angle of Incidence.

Angle of Reflection Formula

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The Angle of incidence always equals the angle of reflection and the distance of image always equals the distance of object are called Angle of reflection formula.

if $\theta_{i}$ and $\theta_{r}$ are the angle of incidence and angle of reflection respectively, then
sin $\theta_{i}$ = sin $\theta_{r}$or
$\theta_{i}$ = $\theta_{r}$
This is the Angle of Reflection Formula.

Grazing Angle

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Now, in the same diagram, we see another ray BO which falls on the mirror and reflected as as a ray OB’. In this case, it is easier to perceive the angles at which the incident ray and the reflected ray with the surface of the mirror rather than to its normal. They are angles ‘r’ and ‘s’ and these are called ‘grazing angles’. The angles ‘r’ and‘s’ are congruent because of the fact they are the difference of 90o – the angle of incidence or reflection as the case may be.
When we look at a plane mirror we see our image as a reflection, as if you stand behind the mirror at the same distance.
This is only ‘as if’, because practically ‘behind the mirror’ there is nothing. If you keep a screen behind the mirror at the same distance, no image will be formed on the screen. That is the image is ‘virtual’ and not real. Images formed by plane mirrors are always a virtual image.

Plane Mirror
Plane mirrors are the mirrors which form the real images on by undergoing the phenomena of reflection.These mirrors will be having plane surface. 
→ Read More Mirrors can be also in curved shapes. Basically the curves are parabolas and hence curved mirrors are also called as parabolic reflectors. Under the category of curved mirrors, there are two types:
  1. Concave Mirror
  2. Convex Mirror.
Curved Mirror
The mirror shown on the right is called a convex mirror and it is exactly opposite in shape and characteristics to a concave mirror.
In both cases, the axis of symmetry is the principal axis of the mirror and the point F is the focus of the curved mirror. The length OF is known as focal length of the mirror.

Let us know the about these both mirrors.
  1. Concave Mirror : A concave mirror can produce a real images in the same direction or in the inverted direction depending on the position of the object. When a light source is placed on the focus of the mirror, the reflected rays will be parallel. This feature is used in torches to produce beam of light.
  2. Convex Mirror : In case of convex mirrors, since the focus is on the other side of the object, the mirror reflects the rays from the object outwards. Therefore, convex mirrors can only produce virtual images. Convex mirrors are extensively used as viewing mirrors in automobiles.
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Diffused Reflection

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When light falls on the smooth and polished surface it bounces back. This phenomena is called as reflection.

The reflection are of two types:
  1. Specular or regular reflection
  2. Diffused or irregular reflection.
In the case of regular reflection, it obeys the laws of reflection completely but in case of the diffused reflection, it does not obey the laws of reflection completely. The diffused reflections are done by the non-polished or the slightly polished and non smooth or rough surfaces.
                                               Diffused Reflection
                                       Figure above represents the light falling over the rough surface

As the rays of light falls on a rough and non-polished surface at any angle of incidence then the angle of reflection is not equal to the angle of incidence. Here the rays of light do not follow the first law of reflection. Similarly, the normal to the reflecting surface, incident ray and the reflected ray are not lie in the same plane, so the rays of light do not follow the second law of reflection. 
More topics in Reflection
Law of Reflection Angle of Incidence
Plane Mirror Curved Mirror
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