PLANE MIRROR REFLECTION

                                                   PLANE MIRROR REFLECTION

INTRODUCTION: Light is a form of energy which excites the sensation of light. We see an object because of the light it reflects. A continuous and  great experiment  proved that light always  travels in a straight line under an experimental evidence , for example, a  bright source of light like the sun casts distinct shadows ; the beam of torch appears to be in a straight line.Light travels at 299,792,458 ms-1 or 3×10 8 ms-1. It takes 8 minutes for a light wave ( or a photon) to travel form the sum to the earth. We see the moon because it reflects the sunlight. Light takes 1 second to reach the earth when reflected back by the moon.

 

PHENOMENON OF LIGHT: When a beam of light travelling through a medium, is incident on any other surface, three situations can arise.

  • A part of light, on striking the surface, is sent back in the original medium. It is called reflection of light.
  • A part of light, on striking the surface, gets transmitted through it. It is called refraction of light.
  • A part of light, gets absorbed by the medium.

 

REELECTION OF LIGHT:The phenomenon due to which a beam of light, travelling througha certain medium, on striking some other surface, bounces off in some other direction, this phenomenon us known as reflection of light.

In other words, the bouncing back of light when it strikes a smooth or polished surface is called reflection of light. While studying the reflection of light we consider light to be a ray.A ray of light represents the direction of the propagation of light i. e. light travels in the direction of the ray of light. Reflection is of two types:

  • Regular reflection: When several rays of a beam of light incident on a smooth, mirror like reflecting surface. In this reflection the rays which are reflected from a body are always parallel. Reflection of light from such a smooth surface is called specular reflection or regular reflection.
  • Irregular reflection: The phenomenon due to which a parallel beam of light, travelling through same medium gets reflected in various possible direction on striking some rough surface is called irregular reflection or diffused reflection.                                                                                      If the reflecting surface is rough, the surface will reflect the rays in various directions .The reflection which takes places from the ground,walls, trees, suspended particles in air and a variety of other objects which are not very smooth, is irregular reflection. This type of reflection helps in spreading light energy over a vast region and also decreases it’s intensity.Thus, it helps general illumination of places and helps us to see things around us.

 

TERMS CONNECTED WITH REFLECTION OF LIGHT

 

TERMS DEFINITION DIAGRAM
Incident ray It is a light ray which strikes the reflecting surface.
Reflected ray It is a light ray which bounces back into the same medium after reflecting from the reflecting surface.
Point of incidence it is a point on the reflecting surface where the incident ray strikes.
Normal It is a perpendicular drawn at the point of incidence on the reflecting surface.

Angle of incidence

It is the angle which the incident ray makes with the normal to the reflecting surface at the point of incidence. It is usually represented by the letter <i.
Angle of reflection It is the angle which the reflected ray makes with the normal to the reflecting surface at the  point of incidence. It is usually represented by the letter <r.
Glance angle of incidence The angle which the incident ray makes with the mirror is called glance angle of incidence. <MOA is that angle.
Glance angle of reflection The angle which the reflected ray makes with the mirror is called glance angle of reflection. <BOM is that angle.

 

LAWS OF REFLECTION

  • The incident ray, the reflected ray and the normal lie in the same plane , at the point of incidence.
  • The angle of incidence is always equal to the angle of reflection.

A light ray is considered which is travelling in air and incident at an angle on a flat , smooth surface. The incident ray is AO and the reflected ray is BO ; these the rays make angles i and  r respectively, with a line ON drawn perpendicular to the surface at the point of incidence O. Then experiments show that

<i= <r.

A PLANE MIRROR:

A plane mirror is a plane reflecting surface. Image formed by a plane mirror can be studied under two different headings

  • WHEN THE OBJECT IS A POINT OBJECT

O is the point object, MM’ is the plane mirror. O is kept at u distance from mirror. Image I is formed at the point from where they appear to originate.The rays appears to originate at I, which is at a distance v behind the mirror, this is the location of the image.

The image seen in a plane mirror is always virtual, erct and same size. This image cannot be displayed on a screen.

 

  • WHEN THE OBJECT IS AN EXTENDED OBJECT

PS is an extended object kept infront of the mirror MM’ at u distance. Two rays are originated from the object PQ and PR. Anobserver to the left of the mirror would trace the two reflected rays back to the point from which they appear to have originated at the point P’. Son it can be concluded that the images formed by an object in front of mirror. It can also be shown with the help of geometry, that the object height h  is equal to the image height h.

Plane mirror forms this type of image:

  • Image is virtual
  • Image is erect
  • mage and object are having same size.
  • Image is formed as far ehind the mirror
  • mage is laterally inverted.

 

INVERSION AND LATERAL INVERSION OF THE IMAGE

During inversion the top side of oject appears as bottom side of image and vice versa. In a way the image turns around horizontal through an angle of 180°.

 

 

During lateral inversion the left side of the object appears as right side of image and vice versa. Ina way the image turns through an angle of about 180° vertical axis.

 

TO PROVE GEOMETRICALLY THAT IMAGE FORMED Y A PLANE MIRROR IS A FAR EHIND THE MIRROR, AS THE OBJECT IS INFRONT OF IT —–

O is an object  situated infront of a plane mirror MM’. A ray of light which strats from point O perpendicularly, is reflected back along the same path. Another ray which moves along OB, is reflected along BC, obeying the laws of reflection , such that BN is the normal. OA  and OB

are produced backward in such way that they meet at the point I.  Then I is the image of O. We have to prove that OA = IA.

Statement Reason
<i = <r By the laws of reflection
<i = <1 Pair of alternate angles
<I = <2 Pair of corresponding angles
so <1 = <2 By comparing.

In ΔBAI and ΔBAO

<1 = <2

<3 = <4                                                                                   {Each angle is 90°}

BA = BA                                                                                  Common

                                                                             

So ΔBAI ≡  ΔBAO

By c.p.c.t.

OA = IA

Thus in a plane mirror  the image formed as far behind the mirror, as the object is in front of it.  

                                               

 

 

 

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