Formation of image in a plane mirror - Param Himalaya

Formation of image in a plane mirror ( Point Object ) : 

1. Consider a small object O ( say , a point source of light ) placed in front of a plane mirror MM'. 
2. Now , a ray of light OA and OB coming from the object O is incident on the plane mirror at point A and B.
3. It's reflected in AX and BY according to the law of reflection of light.
4. The mirror will form an image I of the object O.The two reflected rays AX and BY are diverging ( Moving away from one another ) so they can't meet on the left side . 
5. Let us produce the reflected rays AX and BY backwards . They meet at point I behind the mirror. 
6. Thus point I is the image of the object O formed by the plane mirror . 
7. The image formed by a plane mirror can be seen only by looking into mirror. 
8. An image of this type which cannot be received on a screen is known as a virtual image.

Formation of image in a plane mirror ( an extended or finite object) : 

1. Most of the actual objects are much bigger than a mere ' point ' and they are called ' Extended objects ' or 'finite object'.
2. We will use an ' arrow pointing upwards to represent an extended object ( or finite object ).
3. The nature of image formed by a plane mirror is virtual and Erect.
4. The size of image formed by a plane mirror is equal to that of the object.

The position of Image formed in a plane mirror : 

1. The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of the mirror. 
2. If an object is placed at a distance of 5 cm in front of the plane mirror , then it's image will be also be formed at a distance of 5 cm behind the mirror.
3. Total distance between the object and it's image will be 5+5 = 10 cm.

Lateral Inversion : 

When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image , and the left side of object appears to become the right side of images. 

This change of sides of an object and it's mirror image is called Lateral inversion.

• The phenomena of lateral inversion is due to the reflection of light .
• The image formed in a plane mirror is laterally inverted ( or side ways reversed ) with respect to the object. 

Relative motion of object and image :

Case 1. If an object moves towards ( or moves away ) from a plane mirror with speed v , it's image will also approach ( or move away ) with speed v.

So, the speed of the image relative to the object will be v-(-v) = 2v . Therefore, it appears that the object and the image are moving away or approaching each other with a relative speed of 2v.

Case 2. If the mirror is moved towards ( or away from ) the object with speed v then the image will move towards ( or away from ) the objects with a speed '2v'.

Multiple Reflection:

Number of images formed by combination of two plane mirror depends on the angle between the mirrors.

If 0 ( in degrees) is the angle between the plane mirror then number of image formed are given as , 

n = 360/0 - 1

If there are two plane mirrors inclined to each other at an angle 90°, the number of images of a point object is equal to 3.

Deviation:

Deviation is defined as the angle between directions of incident Ray and the emergent ray. So if light is incident at an angle of incidence i, 

A = 180°- ( i +r = 180 -2i 

So , if incident at angle of 45° ,

Deviation = ( 180°- 2 ×45° = 90 

And for normal incidence i = 0 a = 180°

Use of Plane mirror:

1. Plane mirrors are used to see ourselves.
2. The mirror on our dressing table and in bathroom.
3. Plane mirrors are used in making Periscope.
4. They are used in solar cookers.
5. They are also used in various scientific instruments.