9th. Reflection of light

 Reflection of light

Light is actually electromagnetic radiation which causes the sensation of vision.

Definition of mirror :- Surface which reflects light  and create clear image is called a mirror.

Mirrors are of two types plane mirror and spherical mirrors.

Spherical mirrors are of two types

 A. concave mirror, 

 B. convex mirror.

How is the mirror prepared from class?

 A mirror is made by coating the back surface of a flat and smooth glass piece with a thin reflecting film of aluminium Al or silver Ag , to protect this reflecting field and to make that side opaque ( not letting  light through) another cout of substance like lead oxide given over it.

Real image :- Real image is always formed by convex lens. One that can be formed on a screen by the actual light rays meet each other to form an image, it is called a real image, 

Virtual image :- When the reflected rays without actually meeting create image it is called as virtual image. Virtual image cannot be taken on screen.

The image formed by a plane mirror is of the same size as the source.

The mirror formula 

n = number of images,

A = angle between the mirror

If two plane mirror are placed at a certain angle, the number of images obtained is as follows

Angle.   |     Number of images

------------|---------------------------------

30.         |    11

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45.         |     07

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60.         |     05

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90.         |     03

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120.       |     02

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When can we see our full image in the mirror?

To see the full image of a person standing in front of a mirror the minimum height of the mirror half the height of the person.

Terms related to spherical mirrors

1. Convex mirror :- when the outer surface of the spherical mirror is reflecting then it is called convex mirror.

2. Concave mirror :- When the inner surface of spherical mirror is reflecting surface than it is called a concave mirror.

3. Pole :-  The centre of the mirror is called its pole P.

4. Centre of curvature :- The centre of the sphere of which the mirror is a part is called the centre of curvature C.

5. Radius of curvature :- The radius of the sphere of which the mirror is a part is called the radius of curvature R

6. Principal axis :- The straight line which passing through the pole and the centre of curvature of the mirror is called its principle axis.

7. Principle focus :- 

A. Principle focus of concave mirror :- The rays of light which are parallel to the principal axis after reflection meet at a point on principle axis is called principle focus of concave mirror F.

B. Principal focus of convex mirror :- The rays of light which are parallel to the principal axis after reflection appear to come from a point behind the mirror on principal axis is called principal focus of convex mirror.F

8. Focal length :- The distance between the pole P and the principle focus  F of the mirror is called focal length f.

Above terms can be understand easily by following figure.

Always remember that focal length is half of the radius of curvature of the mirror.

 What are the rules used in drawing ray diagrams by spherical mirrors?

1. Rule 1. :- The ray of light which is parallel to the principal axis after reflection it  passes through the principal focus. 

2. Rule 2  :- The ray of light which passes through the principal focus after reflection it is parallel to the principal axis.

3. Rule 3 :- If an incident ray passes through the centre of curvature C then the reflected ray traces the same path back.