What is a spherical mirror for Class 7?

Mirror :          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.

Plane Mirror:

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.

What is a spherical mirror for Class 7?

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.

    
Note :     
    (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. 

What is a spherical mirror for Class 7?

(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. 

What is a spherical mirror for Class 7?

Some terms related to Spherical Mirrors : 

What is a spherical mirror for Class 7?

(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.

What is a spherical mirror for Class 7?

(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.

What is a spherical mirror for Class 7?

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.

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There are two types of mirrors – plane mirrors and curved mirrors. If the curved mirror is a part of a huge sphere, then the mirror is a spherical mirror. Spherical mirrors can be further classified into two types – concave mirrors and convex mirrors.  Like plane mirrors, spherical mirrors also follow the laws of reflection. 

Terms Associated with Spherical Mirrors

  • Centre of curvature (C) is the centre of the sphere, of which the mirror is a part.
  • Radius of curvature (R) is the radius of the sphere, of which the mirror is a part.
  • Pole (P) is the geometric centre of the spherical mirror.
  • Principal axis is the line joining the pole and the centre of curvature.
  • Principal focus (F) is the point on the principal axis, where a parallel beam of light, parallel to the principal axis after reflection converges in the case of a concave mirror and appears to diverge from in the case of a convex mirror.
  • Focal length (f) is the distance of the principal focus from the pole of the mirror.

                            

What is a spherical mirror for Class 7?
There are two types of images: real and virtual. Real images are those that can be caught on a screen while virtual images are those that cannot be caught on a screen.

Concave Mirror

If a part of a hollow glass sphere is cut and the cut part of the sphere is coated outside with silver or similar material, then its inner surface reflects the entire light incident on it, and thus, forms a mirror. Since the inner surface is a concave surface, the mirror so formed is called a concave mirror. Concave mirrors converge the light incident on them and hence are called converging mirrors. We can observe ourselves magnified when the mirror is placed close to our face. This is due the position of the object between the focus and the pole. As the object moves away from the mirror, the size of its image reduces along with its distance from the mirror. If an object is placed close to a concave mirror such that the distance between the mirror and the object is less than its focal length, then a magnified and virtual image is formed. Due to this property, concave mirrors are used as shaving mirrors, and by dentists to view clearly the inner parts of the mouth.

Convex Mirror

If the cut part of the glass sphere is coated from inside with silver or a similar material, then its outer surface reflects the entire light incident on it, and thus forms a mirror. Since the outer surface is a convex surface, the mirror so formed is called a convex mirror. Convex mirrors diverge the light incident on them and hence they are called the diverging mirrors. Due to this they always form diminished, virtual and erect images irrespective of the position of the object in front of them. Thus, the magnification produced by these mirrors is always less than one. The field of view for a convex mirror is greater than that for a plane mirror, the aperture being the same. Hence, convex mirrors are used as rear-view mirrors in vehicles. It is also installed behind automated teller machines as a security measure. 

Rules for Construction of Ray Diagrams for Spherical Mirrors

Rule 1: A light ray incident parallel to the principal axis, after reflection, either actually passes through the principal focus or appears to pass through the principal focus.


Rule 2: A light ray which first passes through the principal focus or appears to pass through the principal focus, after reflection, will travell parallel to the principal axis.  
Rule 3: A light ray which first passes through the centre of curvature or appears to pass through the centre of curvature, after reflection, retraces its initial path. 

Image Formation by Concave Mirror

Depending on the position of the object in front of the concave mirror, the position, size and the nature of the image varies.

1. Object at infinity:  A real, inverted, highly diminished image is formed at the focal point F, in front of the concave mirror.

What is a spherical mirror for Class 7?

2. Object beyond C: A real, inverted, diminished image is formed between C and F, in front of the concave mirror.


 

What is a spherical mirror for Class 7?

3. Object at C: A real, inverted, same sized image is formed at C, in front of the concave mirror.

What is a spherical mirror for Class 7?

4. Object between C and F: A real, inverted, enlarged image is formed beyond C, in front of the concave mirror.

What is a spherical mirror for Class 7?

5. Object at F: A real, inverted, highly enlarged image is formed at infinity, in front of the concave mirror.

What is a spherical mirror for Class 7?

6. Object between F and P: A virtual, erect and enlarged image is formed behind the concave mirror.

What is a spherical mirror for Class 7?

Image Formation by Convex Mirror

Irrespective of the position of the object, a virtual, erect and diminished image is formed between F and P, behind the convex mirror. 

What is a spherical mirror for Class 7?

Mirror Formula and Sign Conventions 

The relation between the focal length (f), object distance (u) and the image distance (v) is given by 1/f = 1/v + 1/u. This is called the mirror formula. All the distances are measured from the pole of the mirror. If we measure the distances in the direction of the incident light, then they are taken positive or else they are taken negative. These constitute the sign conventions.

Uses of Concave Mirrors 

  • Concave mirrors are used as shaving mirrors to see a larger image of the face.
  • Dentists use concave mirrors to view the back of the tooth.
  • ENT doctors use them for examining the internal parts of the ear, nose and throat.
  • They are used as reflectors in the headlights of vehicles, search lights and in torch lights to produce a strong parallel beam of light.
  • Huge concave mirrors are used to focus sunlight to produce heat in solar furnaces.


Uses of Convex Mirrors

  • Used as rear view mirrors in automobiles as it covers wide area behind the driver.
  • Used as reflectors for street light bulbs as it diverges light rays over a wide area.