7th Class Science Lights and Sound Spherical Mirrors

Spherical Mirrors

Category : 7th Class

*       Spherical Mirrors

 

The name of the spherical mirror itself indicates that it is the part of the sphere. There are two types of spherical mirrors, convex and concave mirror. The reflective side of the concave mirror bulges away from the observer and in the convex mirror it bulges towards the observer.

For the better understanding of the concave and convex mirror, a tablespoon may be taken as an example. The inner surface of the table spoon is the shape of concave and the outer surface is the shape of convex.

 

Look at the following pictures of convex mirror

                      

Fig.(1) Virtual image (In convex mirror)                                       Fig.(2) Index of a convex mirror    

 

      Fig. (3) Image formed by convex mirror                

In the picture (1), a virtual image is shown by a convex mirror. Convex mirror is used in automobiles to get the wider view of the automobile. The curved surface of convex mirror gives the wider and clear view of the object approaching. It is used in departmental stores to get the wider view of the inside of store. Convex mirror reflects light outwards. It cannot be focused on an object, therefore cannot be used to focus light. The center of curvature and focus, both are imaginary. Therefore, they form virtual image. The center of the shiny image of the convex mirror is called vertex and denoted by V. When parallel light rays strike on the convex mirror, it gets reflected. Virtual image is formed at the imaginary intersecting point of the reflected rays, as shown in the figure (3).

 

Look at the following pictures of concave mirror

                                      

      Fig. (1) Real image                                                        Fig (2) Index of a concave mirror  

 

Fig (3) Image by a concave mirror  

In the figure (1), an image is shown formed by a concave mirror. A real image is formed by the concave mirror when object is placed in front of the mirror. A concave mirror reflects light and reflected rays of light intersects each other outside the mirror, therefore real image (not virtual) is formed.

The center of curvature is the center of sphere and denoted by C, radius of the curvature is denoted by r. The center of the shiny surface of the mirror is called the pole of the mirror and it is denoted by P. In the figure (3), incident rays are reflected and meet at a point called focus .which is denoted by F. Focus, centerof curvature and pole of the mirror lie on the same plane,which is called principal axis, as shown in the figure (1). The distance between the pole and focus is called focal length. Focus of a concave mirror is exactly in the middle of the center of curvature and pole.

 

*           Formation of Image by a Convex Mirror

If the distance between the object and the mirror is changed, the characteristics of formed image also change. Let us know about the characteristics of images when object is placed at different distances from the mirror.

 

*            When Object is Between Infinity and The Pole of The Mirror

The ray parallel to the principal axis strikes to the mirror and gets reflected. On producing it behind the mirror, it intersects the principal axis at F. The secondary ray strikes the mirror and retraces along the same path. On producing it behind the mirror, it intersects the principal axis at C. The image is formed at the imaginary meeting point of these reflected rays, as shown in the picture below. The image is smaller than the object, virtual and erect.

               

 

*            When Object is at The Infinity

When object is at infinity, the rays of light, coming from the object, are parallel to each other. The parallel rays are diverged after the reflection by the concave mirror. If the reflected rays are produced behind the mirror, they converged at the focus. And here an image of the object is formed. The image is highly diminished, virtual and erect.

   

 

*             Formation of Images by a Concave Mirror

Let us know about the characteristics of images when an object is placed at different position in front of a concave mirror.

 

*              When Object is at the Infinity

In a concave mirror, when object is at the infinity, the parallel rays of light coming from the object converged at the focus after reflection by the mirror. Thus image is formed at the focus of the concave mirror. The image is highly diminished, real and inverted.

 

 

*           When Object is beyond the Centre of Curvature

When the object is placed beyond the center of the curvature (c), the parallel (with respect to principal axis) ray strikes on the mirror and intersects the principal axis at the focus. The another ray, which passes through the focus, does not intersect the principal axis after reflection. The point of intersection of the reflected rays lies between C and F. Thus image is formed between C and F. The image is smaller than the object, real and inverted.

 

 

*           When Object is at the Centre of Curvature

When object is placed at the center of the curvature, the point of intersection of reflected rays lies just below the centre of curvature. Thus the object is formed at the centre of curvature of the mirror. The image is of same size as the object, real and inverted.

 

 

*           When Object is Between Centre of Curvature and Focus

Magnified image is formed when an object (ST) is placed between the center of curvature (c) and focus (F). The point of intersection of the reflected rays lies beyond the centre of curvature. Thus the image is formed beyond the centre of curvature. The image is enlarged, real and inverted.

   

 

*           When Object is at the Focus

When the object is placed exactly at the focus, the incidents rays get parallel to each other after the reflection by the mirror. Therefore, image of the object forms at the infinity. The image is highly enlarged, virtual and erect.

 

 

*            When Object is Between The Pole and The Focus

When an object is placed between the pole and the focus, the incident rays, after reflection by the mirror, moves away of each other. The point of intersection of reflected rays we get on producing them behind the mirror. Thus image is formed behind the mirror. The image is enlarged, virtual and erect.

 

 

 

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          Which type of mirror will make a virtual and magnified image, if an object is placed in front of the mirror, between the focal point and pole.

(a) Plane mirror

(b) Convex mirror

(c) Concave mirror

(d) All of these

(e) None of these

 

Answer: (c)

 

 

         If distance of centre of curvature and focus of a spherical mirror is A m, find the distance between focus and the mirror.

(a) 2A m

(b) A/2 m

(c) 3A m

(d) A/4 M

(e) None of these

 

Answer: (b)

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