- Books Name
- CBSE Class 7 Science Book
- Param Publication
- CBSE Class 7
It is a highly polished surface, which is quite smooth and capable of reflecting a good fraction of light from its surface.
(a) Object :
Anything which gives out light rays (either its own or reflected) is called an object.
(b) Image :
The reproduction of object formed by mirror or lens is called an image.
ReaL and virtual images :
The image which can be obtained on a screen is called a real image. It is formed when light rays, after reflection, actually intersect each other. It is always inverted.
The image which cannot be obtained on a screen is called a virtual image. It is formed when light rays, after refection, intersect when extended in backward direction. Its is always erect.
Image Formation by Plane Mirror :
Consider a point source of light placed at a point O at a distance u in front of the plane mirror. Light rays leave the source and are reflected from the mirror. After reflection, the rays diverge but they appear to come from a point I located behind the mirror. Point I is called the image of the object O. Point I is at a distance v behind the mirror.
Character Stics of Image :
(i) Image is virtual, erect and of the same size as object.
(ii) It is as far behind the mirror as the object is in front of it
(iii) Laterally inverted : When left appears right and right appears left.
(i) Minimum size of the mirror required to see full image of a person is at least half of his own height.
(ii) If object moves with a speed V towards plane mirror then image moves with a speed 2V towards object.
(iii) If mirror moves with a speed V towards stationary object then image moves with a speed 2V towards object.
Uses of Plane Mirror
* In looking mirrors, hair saloons, reflecting periscopes & kaleidoscopes.
Spherical Mirrors :
Mirrors, whose reflecting surfaces are spherical or curved, are called spherical mirrors. These are of two types:
(i) Concave mirror: If the reflecting surface of the spherical mirror is curved inwards, it is called a concave mirror. The image formed by a concave mirror can easily be taken on the screen. You must have played with concave mirror to obtain image of sun on the ground or wall by positioning it to in a specific way. It means a concave mirror can form real or virtual image. In concave mirror when a parallel beam of light after reflection from a concave mirror converges at a point in front of the mirror. Due to this it is known as converging mirror.
(ii) Convex mirror: If the reflecting surface of the spherical mirror is curved outwards, it is called a convex mirror. It means a convex mirror always forms virtual image. In convex mirror when a parallel beam of light after reflection from a convex surface diverges and the rays do not meet. Due to this it is known as diverging mirror.
Some terms related to Spherical Mirrors :
(i) Pole : The central point of a mirror is called its pole.
(ii) Centre of curvature : The centre of the sphere of which the mirror is a part, is called centre of curvature.
(iii) Radius of curvature : The radius of the sphere of which the mirror is a part, is called radius of curvature.
(iv) Principal axis : The straight line joining the pole and the centre of curvature is called the principal axis.
(v) Focal length : The distance between the pole and the focus is called the focal length. The focal length is half of the radius of curvature.
(vi) Aperture : The size of the mirror is called its aperture.
(vii) Focus point :
(a) In concave mirror when a parallel beam of light after reflection from a concave mirror converges at a point in front of the mirror. This point (F) is the focus of a concave mirror and it is real.
(b) when a parallel beam of light after reflection from a convex surface diverges and the rays do not meet. However on producing backward, the rays appear to meet at a point behind the mirror. This point is focus of the convex mirror and it is virtual.
Uses of Concave Mirror :
(i) Concave mirror is used by doctors for examining eyes, ears, nose and throat.
(ii) They are also used by dentist to see an enlarged image of the teeth.
(iii) The reflector of torches, headlights of cars and scooters are concave in shape.
Uses of Convex Mirror :
(i) Convex mirrors are used as rear view mirrors to see the vehicle coming from behind. These mirror covers a wider view and form upright small size (diminished) image.
(ii) They are used as reflector in street lamps so as to diverge light over a large area.