8th Class Science Light Formation of Image by Plane Mirror

Formation of Image by Plane Mirror

Category : 8th Class

*       Formation of Image by a Plane Mirror


Incase of the plane mirror, the distance of object and image formed are equal,but are on the opposite side of the mirror. One of the important facts observedin the plane mirror is that, when an object is placed in front of a plane mirror,the right side of the object appears to be left and the left side appears to beright. This phenomenon is called lateral inversion.


The word AMBULANCE on the hospital van is always written in the form of itsmirror image. This is done because if we are driving a car, we will be able tosee the hospital van coming behind us, in rear view mirror. We get the laterallyinverted image of the word ambulance and thus give pass to the van.



*         Uses of Plane Mirror

(a) It is used as a viewing mirror in our house

(b) Is used in the shop of the barber, jeweler etc

(c) It is used in periscope


*          Spherical Surfaces

There two types of spherical mirror:

(i) Concave

(ii) Convex

Concave Mirror: The mirrors curved inwards are called concave mirror.

Convex Mirror: The mirrors curved outwards are called convex mirror.

The spherical surfaces consist of principal axis, focus, poles, centre of curvature.


The straight line passing through the focus and the pole of the mirror is calledthe principal axis.


*           Focus

The point on the principal axis at which the ray of light converges after reflectionfrom the spherical surfaces is called principal focus.


*           Focal Length

The distance between the focus and pole is called the focal length.


*           Pole

The point at which the principal axis passes the mirror is called the pole.


*           Centre of Curvature

The centre of curvature of the mirror is the centre of hallow spherical surface,of which the mirror is the part. It lies in front of the mirror, and on the principalaxis.


*         Radius of Curvature

The distance between the centre of curvature and the pole is called the radiusof curvature.


*           Aperture

The portion of the mirrorfrom which the reflection of light actually takes place,or it is the diameter of the reflecting surfaces.


*            Relation between the Radius of Curvature and FocalLength

For any spherical surfaces, focus is always the mid-point of the centre ofcurvature and the pole. Thus, if F denotes the focal length and R is the radiusof curvature, the relation is given by,



Image Formation in Concave Mirror

Position of object Position of Image Size of Image Nature of Image
(a) Between pole and F Behind the mirror    Enlarge             Virtual and erect
(b) At F                At infinity           Highly enlarge      Real and inverted
(c) Between F and C    Beyond C           Larger in size       Real and inverted
(d) At C               At C               Equal in size        Real and inverted
(e) Beyond C           Between F and C    Smaller in size      Real and inverted
(F) At infinity           At F                Highly diminished   Real and inverted




*            Uses of Concave Mirror

(a) A concave mirror can be used as a reflector in search light, vehiclehead light, and torches.

(b) It is used as a saving mirror to see the large image of the face.

(c) It is used by dentist to see the large image of teeth.

(d) It is used as a reflector in the solar furnace and many other solardevices.


*            Mirror Formula

If U is the object distance, V is the image distance and F is the focal length,the relation between them isgiven by:



*             Magnification

It is defined as the extent to which an image can be magnified. It is given by,


Here, \[{{h}_{1}}\]and\[{{h}_{2}}\] are the height of the object and height of the image respectively.


*           Uses of Convex Mirror

It is used as a rear view mirror in cars, trucks, and buses, as it produces anerect image. It also gives wide range of visibility.





       The image obtained is either erect or inverted. The image is either infront of the mirror or behind the mirror. If the it is a concave mirror,in that case the erect image will be____.

(a) Real

(b) In front of mirror

(c) Virtual

(d) below the principal axis

(e) None of these


Answer: (c)



       Different types of images are obtained from the different position ofobjects. John takes a concave mirror and places the object at a distancetwice that of focal length. Where will be the image formed?

(a) At infinity

(b) Beyond C

(c) At C

(d) Between C and F

(e) None of these


Answer: (c)


Twice the focal length is centre of curvature, called the radius ofcurvature. If the object is placed at curvature, the image will be formingat curvature itself.

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